Tried some other things just in case. Updated bios (apparently wasn't on the most recent) and forced PCI to gen 3. Played a game for a while with no problem, but then just blackscreened on the desktop again, however this time it actually recovered. Checked the event viewer, and found a couple of 'Error 4101: Display driver amdkmdap stopped responding and has successfully recovered.'
I have my old i7 rig I could test it in, though testing with that would be a pain because I don't have the stock intel heatsink, just the noctua that I'm using in my new rig. Might have to resort to that though, good thinking.
You're basically having the same issues I've been having with my V64 on and off for months.
Watching Twitch, Youtube etc on one screen while doing something on the main screen, could be gaming, could be browsing web sites or messages in Discord, or nothing at all.
And it'll randomly have an issue, sometimes it's fine for days at a time, some days I can get multiple issues.
Similar to what you've described there are various 'levels' to it but I think they are all the same thing just various levels of how well the drivers and the o/s deal with it.
Black screen followed by recovery, driver crashes/resets. Best case, rarest one.
Blue screen with the TDR (116) issue. Mem dump/Reboots.
Black screen, appears to be locked up but checking the HDD light on the case it is blinking. Leave it and it eventually reboots and TDR issue in the event logs. So basically it is blue screening and writing a dump successfully but no display.
Black screen, rare times it can be grey, green, what ever. Total lock up, no HDD activity, leave for a while no reboot, manually have to trip the power.
V64 under an EKWB modular water cooler.
i7 3770K , 16GB mem.
Win10 Pro , always updated to the latest.
Various AMD drivers.
Screen 1: DP 1440p@144Hz freesync
Screen 2: DP 1080p@120Hz
Screen 3: HDMI 1080p@60Hz to a Yamaha a/v amp using Dolby Atmos, and then hooked through to a Samsung TV 2160p@60Hz for the odd bit of couch gaming.
Not always had these issues and the same set up was basically fine with an old AMD Fury in it.
But at some point either the drivers and/or Windows changed and these issues have come and gone and definitely seem worse on the latest AMD drivers.
TBH I think a lot of the issues are still down to when non-gaming desktop apps use h/w acceleration at the same time and you flip between them, changing their load but also it just not coping well with multi displays at different res/refresh.