I am running KDE Plasma 5.18.8 on Ubuntu 20.04.4 LTS (installed from Ubuntu Server ISO) with NVIDIA 510 drivers. I have 3 identical 240hz monitors running on display port from a RTX 2070 Super.

I have been running into an annoyance where approximately 10 minutes after being logged in, my primary monitor will go blank for a few seconds and come back on. Then < 1 minute later my left monitor will do the same thing. The right monitor does not do this.

This only ever happens once per boot, and seems to be consistently ~10 minutes after boot.

I have been searching through /var/log but I can't seem to find anything that correlates to the monitor misbehavior.

Any suggestions on how to debug this?

Edit: I'm trying to recreate it and narrow down exactly the conditions it happens.

Edit 2: I have not been able to reproduce this issue. What is a good way to get more verbose logging that might catch this event if it happens again?

Edit 3: I set X to start logging with a higher verbosity level by creating a file /etc/sddm.conf.d/custom_settings.conf with the contents:

[X11]
ServerArguments=-logverbose 6 -verbose 2

According to xorg man page:

logverbose (stdout) deafult is 3

verbose (stderr) default is 0

Neither specify how many levels there are in the man page. I found 6 from other stack exchange questions for -logverbose and chose an arbitrarily higher number for -verbose.

It could be two problems AFAIK

Signal Interference

DisplayPort is prone to this because it has got "Link Training" which means the GPU and Monitors will negotiate how fast the link can go; and then start transmitting if the noise is acceptable for the amount of bandwidth required. Link Training can trigger at any time if the conditions changed (usually due to an EMF spike, because there's dropped packets, but also because SW changed resolution, frequency, colour space, etc)

Your situation sounds to be quite extreme because 240hz requires a lot of bandwidth.

You may be running into EMF interference which is documented and definitely a thing: Display intermittently blanking, flickering or losing video signal.

Office chairs are known sources of EMF which can trigger this situation.

Recommendations to fix this issue is to use shorter cables, and high quality ones. I don't know if magnetic ferrite rings will help because they filter higher frequencies, but when you're trying to reach 240hz that can be a problem.

Power Saving

Another completely unrelated issue could be power saving. A GPU that enters a low power state may be unable to catch up with all 3 monitors at 240hz each. You can easily test this theory by forcing the GPU to remain in high power state at all times.

If that fixes the problem, then try to tweak the power profile until you find a good balance between GPU power saving and monitors not going off.