I was messing around with dos box, and went full screen, and it completely screwed up all of my displays and basically mirrored them and all my icons and windows ended up all over the place. Made a pretty big mess.

Anyway I managed to fix all of this, but now my second monitor keeps flickering. It will just go black for like 5 seconds and then come back. It was not doing this before this incident with going full screen, so I can't see how it would be a cable or something.

There is nothing in dmesg when this happens.

How would I even begin to start troubleshooting this? For what it's worth I made sure everything is updated to latest but it's still doing it. In fact it got more aggressive now, where it's basically unusable.

OS is Linux Mint 20.1 and GPU is [AMD/ATI] Ellesmere [Radeon RX 470/480/570/570X/580/580X/590] (rev e7) (I think it's the 580 but I don't recall 100%)

It'll be helpful for others to see the make and model of your monitor. Is your monitor a Samsung? I had a Samsung monitor that does this. It'll suddenly power down, as if it's going to sleep, and then power right back on. It'll do this about every five minutes or so. I tried everything I could think of; checked software configs, checked video/power cables, changed ports, etc. Doesn't show up in logs. It was temperature-related too; the more active/warm the monitor was on, the shorter power-reboot/sleep interval it'll do. Down to about a minute. I moved the monitor to a Windows machine to see if it continues there, too. It did. The monitor was more than 10 years old so I wrote it off as simply old age. The "new" monitor I replaced it with did not show this behavior on both computers.

Both are a 28" ASUS Tuf Gaming VG289.

I seem to have found a work around, by setting refresh rate to 50hz for both. Should be 60 though, that's the max it will take and also max the kvm will take.

Worse case scenario I'll leave it like this as it seems to be usable for normal usage but I'll see if ever I try to play video. I found even with 60hz I get tons of jitter.

Oh, and it's connected with HDMI throughout, no adapters of any kind. At work we have all display port and I find it's notorious for causing this exact issue, where 1 monitor will just randomly flicker.

This seems to have stopped with setting both to 50hz.... I guess I will just leave it there. I have like 25ft of cable so I wonder if that is part of the issue. (PCs are in the basement and KVM is upstairs).