I myself dealt with random black screens. screen would just go black for a few seconds and come back. I never experienced reboots, crashes, locks ups, etc.
But after the new build and 2 weeks of trouble shooting it came down to 2 things.
1: If you are using a 4k tv "not a monitor" a tv. make sure in the tv settings you have the hmdi port you are using is set for the correct signal. For example, I have a Sony x85K 4k tv 43" after sometime I found out the hdmi port I was using needed to have the setting on that port set to "VRR" which makes it compatible for pc's. This fixed my first issue of garbled screens when joining games.
2: The ultimate fix was me trying every high quality hdmi cable I could find. Bought 4 of them one was $50. None of them made the black screen go away. Finally after some advice from Hemi on here in the forums, I bought the cable he had. Specs were all the same I been buying. But his was a 6foot hdmi cable. All my issues have now been resolved. Doesn't matter what hdmi cable you get as long as its made for 8k and 4k just don't go over 6ft! Yea, I know... its doesn't fit with your set up and it didn't for mine either but I made it work and moved some things around in the office.
Do these two things and see what you get. Forget all the other try this setting and that quick fix. None of them worked for me and i would be surprised if they did for you.