I'm having some issue. I'm not sure if it is a hardware issue or a software issue, but I'd like to determine the cause of the issue so I can fix it. How can I do that?

WIFI

AP <=> Wifi Access Point, Router

• Neighbor AP is strong, creating contention
  Using a smartphone, download e.g. Wifi Analyzer (free, ads) and let it check your AP against those visible nearby.
  One possible solution to problematic speed or access issues is:
  Make sure to have your AP on a fixed channel, defined to be a "good" choice by the app. Some AP software has trouble swapping channels on "Auto", even though it should do the same change in Auto mode.

• Settings problems
  The easy way out of this situation is to reset the AP and start over. Maybe you changed something in the settings and forgot about it?
  Some AP's has the ability to save settings... if that is possible, and the file turns out to be readable text - when opened with a text editor: save the settings before the reset, then compare with a settings file from after the reset - it might be possible to determine what the problem might have been.

• Hardware problems
  This is a bit harder if there is e.g. broken circuitry in a device.
  Simple things that are possible: Verify all cabling to be fault free, even tiny nicks and kinks may be visible signs of damage that can cause trouble. Swap out one item at a time and try running a fair test of relevant kind.
  Jog all cable ends in and out of the related connector a few times; if it has been in place a long time, there might be oxidization on connector pins, creating "contact resistance" - something that causes electrical signal degradation. If there is a visible antenna; treat it in line with the above: Connection ok? Any damage?

Hard disk

To check the hard drive for disk failures, start the Disks application, select the hard drive on the left, then click the icon in the top right, and choose SMART Data and Self-Tests, and then click Start Self-test and choose the Extended test. This test takes a few hours to run and will give you a large amount of information about the health of the drive.

All of the values start at 100 and work their way down to 0. The terms "old-age" and "pre-fail" are normal. Pay attention to the overall assessment, and to how close the values are working towards the failure point, which is typically 0.

Memtest86

1. As @cocomac wrote in his answer, you can use Memtest86 to test your memory, however he mentioned only the ISO test in his answer, not the memtest package.

2. You can test memory in your running OS with the memtest package.

3. Run the following command to install it:

   sudo apt install memtester
   

4. Running the memory test:

   Here we have to get a little more advanced. Run the free -h command to identify the free memory/unused memory. Then run the following command, which tests 6GB of memory while the rest of the memory is still used by the system OS and services/processes:

   sudo memtester 6G 5
   

   This may take a few minutes (sometimes hours) to complete. It is more optimistic than the ISO memtest86.

Testing the CPU

You can use the following packages to test your CPU:

sudo apt -y install stress-ng s-tui

Run the package:

sudo s-tui

From here use the Down to switch from Monitor to Stress by pressing the Enter key. Now watch the CPU temperatures rise as the system's CPU is tested.

Testing the GPU

First go to this website and install the benchmarking tool.

Click the 'Free Download' button and choose the Linux option in the dropdown. Once the download is complete, there should be a Unigine_Heaven-4.0.run file in the Downloads directory (or the chosen directory).

From a terminal, navigate to the folder with the Unigine Heaven download.

Make it executable:

chmod +x Unigine_Heaven-4.0.run

Then the application can be extracted:

./Unigine_Heaven-4.0.run

Next let's move to the new directory that was created:

cd Unigine_Heaven-4.0/

Now the application can be started:

./heaven

Click the 'Run' button to begin the program.

Thanks to System76 support for letting me use information from their article

Machine Check Exceptions

Machine Check Exceptions are hardware failure events and can be logged with rasdaemon.service to journalctl. On Ubuntu, you can install via:

sudo apt install rasdaemon

verify rasdaemon is active:

systemctl status rasdaemon

Then after the system has crashed or been used for a while take a look at the log:

journalctl -f -u rasdaemon

If there is no log or the log is empty, then the crash isn't related to a hardware failure. The log will stay empty until an MCE happens. Take a look for "uncorrected" errors, as most "corrected" errors can be ignored. If there are a consistent number of "uncorrected" errors, the hardware should be examined.

Using a live USB

First create a live USB using Cocomac's answer:

1. Download the latest (not LTS, and not experimental, but latest and supported) release of Ubuntu Desktop from the Ubuntu website.

2. Verify your download. You don't have to, but I suggest it, especially if your internet connection isn't great. Follow the steps on the Ubuntu Wiki.

3. Next, you need to write the ISO to the flash drive. You can use dd, but because it is easy to mess up, I suggest

   balenaEtcher instead (no affiliation, but I like it). Download the

   Linux 64-bit version. Open a terminal, and navigate to where the

   downloaded .zip is (probably ~/Downloads). Unzip the .zip file with unzip balena-etcher-electron-X.Y.Z-linux-x64.zip. Make the AppImage inside the .zip executable with chmod +x balenaEtcher-X.Y.Z-x64.AppImage. Run it with ./balenaEtcher-X.Y.Z-x64.AppImage.

4. Select the ISO, your flash drive, and hit Flash!. Wait for it to finish, and reboot to your flash drive.

After booting from the USB, click on the Try Ubuntu button. After that, check if the hardware works there. If not, then there is a hardware problem.

Verify if the correct drivers are installed.

Sometimes the correct drivers aren't installed by default. In that case, the system starts using generic drivers. Generic drivers work, but do not give any performance or hardware-only features. In that case, you may want to install the correct drivers.

Run the following command to get a list of devices:

sudo ubuntu-drivers devices

Then install the recommended drivers:

sudo ubuntu-drivers autoinstall 

If the above didn't download the correct drivers then you need to manually search Google and find the driver suitable for your device. The output of sudo lshw will tell you about your vendor and device and, you can install the drivers after you know the correct vendor and device.

If your computer is freezing or crashing you can stress test it to induce the freezing or crashing and generate a report about what happened before the system froze or crashed. stress-ng can stress various subsystems of a computer. It can stress load CPU, cache, disk, memory, socket and pipe I/O, scheduling, and much more. To install stress-ng in all currently supported versions of Ubuntu open the terminal and type:

sudo apt install stress-ng

From the results of man stress-ng :

DESCRIPTION: Go to BIOS settings to make sure that your system is loading in BIOS mode and not UEFI mode. 
       stress-ng will stress test a computer system in various selectable
       ways. It was designed to exercise various physical subsystems of a 
       computer as well as the various operating system kernel interfaces.
       stress-ng also has a wide range of CPU specific stress tests that 
       exercise floating point, integer, bit manipulation and control flow.

       stress-ng  was originally intended to make a machine work hard and trip
       hardware issues such as thermal overruns as well as operating system
       bugs that only occur when a system is being thrashed hard. Use
       stress-ng with caution as some of the tests can make a system run hot
       on poorly designed hardware and also can cause excessive system 
       thrashing which may be difficult to stop.

       stress-ng can also measure test throughput rates; this can be useful to
       observe performance changes across different operating system releases
       or types of hardware, however it has never been intended to be used as
       a precise benchmark test suite, so do NOT use it in this manner.

       Running stress-ng with root privileges will adjust out of memory settings
       on Linux systems to make the stressors unkillable in low memory
       situations, so use this judiciously. With the appropriate privilege,
       stress-ng can allow the ionice class and ionice levels to be adjusted.
       Again this should be used with care.

       One can specify the number of processes to invoke per type of stress
       test; specifying a negative or zero value will select the number of
       processors available as defined by sysconf(_SC_NPROCESSORS_CONF).

Faulty RAM can also be an issue. Faulty RAM is a known cause of freezing. To test RAM select the Memory test option from the GRUB menu while booting.

Immediately after the motherboard / computer manufacturer logo splash screen appears when the computer is booting, with BIOS, quickly press and hold the Shift key, which will bring up a GNU GRUB menu screen. With UEFI press (perhaps several times) the Esc key to get to the GNU GRUB menu screen. Sometimes the manufacturer's splash screen is a part of the Windows bootloader, so when you power up the machine it goes straight to the GNU GRUB menu screen, and then pressing Shift is unnecessary.

memtest86+ is a 16-bit program, and it works in BIOS mode but not in UEFI mode. On computers that have BIOS firmware, you will see a GNU GRUB menu screen that looks like the below screenshot. Select Memory test and press Enter.

On computers that have UEFI firmware go to the BIOS settings, select the Boot tab, and try to temporarily change the settings so that your system is loading in Legacy mode or BIOS mode and not UEFI mode. Then you will be able to select Memory test from the GNU GRUB menu screen.

Discrete graphics processors also have their own RAM. Faulty RAM on discrete graphics processors is a known cause of a special type of freezing while playing videos or playing a game in which the entire screen freezes but an audio loop plays repeatedly and unstoppably until the computer is shut down by holding down the computer's power button.

After you have tried the software solutions temporarily replacing hardware components one at a time is still an option, but there are several caveats. Please read these caveats carefully before temporarily replacing a hardware component.

1. In the case of multiple RAM sticks that are physically accessible to the user, removing one RAM stick at a time until you find the faulty one is an option. RAM sticks should only be swapped with RAM sticks of the same type (e.g. swap DDR3 only with DDR3, DDR4 only with DDR4, etc.).

2. If you have a spare graphics card that is known to be in good condition, swapping the graphics card is an option. Don't just swap in any graphics card that you have on hand. Only swap the existing graphics card with a graphics card that you have used before.

3. Warning: I don't recommend swapping any existing components unless you are sure that the new component is in good condition. Swapping an existing component with a damaged component can also damage your computer.