I've had my 6900 XT for around half a year. I'm late tackling this issue, as it's been misbehaving for that whole time.
The first thing I noticed right after installing was this weird horizontal, green colored glitchy line sometimes appearing at the bottom of my monitor when gaming on a very demanding game (KCD). Apparently that's due to insufficient power - well #$!*. These also appeared in other way less demanding games like Overcooked. At some point they disappeared and now I never see them.
The second thing I noticed was crashes. For 6 months it's been very infrequent, for the past week it's been several times a day. The only game that's been causing me trouble there is WoW. I noticed its fullscreen mode isn't really fullscreen and there's no real way to make it actual fullscreen so maybe that's the reason? Maybe 1 out of every 50 alt tabs will cause a crash. Sometimes it recovers, sometimes it causes a reboot. Interestingly, clicking on "Start Discussion" here with WoW running behind caused a crash. It's like it's out to get me.
The third thing is glitchiness with VLC and youtube. Having 2 vlc open on each monitor and switching from one to the other causes white flashes and a mini freeze. Having youtube open and switching to my second monitor makes it flash and freeze too.
Crucial Ballistix 32 GB (2 x 16 GB) DDR4-3200 CL16
Corsair RMx (2018) 850 W
M27Q monitor (DisplayPort) + generic 1080p monitor (HDMI)
GPU and CPU not overclocked, PSU cords are 2 separate, not 1 daisy chained
Latest drivers, latest bios flashed, no MSI Afterburner but RTSS running at all times
170hz freesync monitor, most apps capped to 167fps with RTSS
I don't keep an eye on temps but they are sitting at 66°C right now on WoW
End of specs ---
What can I do to make this better? Please halp :3
You are right on the limit with an 850w PSU, that is the min recommended.
Is you card an AMD reference card or Sapphire, MSI etc.