Old answer I provided below.
Discovered the problem is driven by Ubuntu trying to use my external monitor. What the flicker is is the normal "hiccup" that happens when you add a new monitor...it just keeps happening over and over.
The problem is well-known, and seems to involve the library used to drive graphics to the screen (including plain old text.
The problem is X-windows (or whatever it is called now)as the library driving the monitors. The more up-to-date package for doing this is "Wayland", which Ubuntu is moving towards making the default.
In the meantime you can fix it by enabling Wayland. To do that:
- edit the file etc/ gdm3/custom.config (you'll need to use sudo to have enough privileges. There is a line that mentions Wayland. By default it is commented out. It says "#WaylandEnable=False.
- uncomment the line by removing "#'.
- change the line to read "WaylandEnable=true".
- Add the second monitor.
This answer was drawn from https://bugs.launchpad.net/ubuntu/+source/xorg/+bug/1883534.
OLD ANSWER:
After a lot of time trying to sort this out, I have my computer stable enough as long as it doesn't turn off.
The tricks were:
rebooting Ubuntu 20 over-and-over, until it completes successfully (and doesn't deliver up error messages, a Grub CLI, or flashing screens). No idea why this is needed.
turning off the auto-off feature. Ubuntu becomes unstable if it goes into auto-off mode, leading to the flashing screen and frozen mouse, non-functioning commands, and loss of IO at times. I simply turned off the auto-off mode that is there to save energy.
Downloaded new drivers (new to me) using CLI commands to allow (for example) the browser to render video. The CLI command I used was "sudo apt install gstreamer1.0-libav".
Deep down I think the only thing that can cause this (errors both at boot and after the kernel is running) is the video card - though I can't definitively prove that.
