Question Black screen and restart after installing new GPU ?

Apr 20, 2021
Hi folks, any advice on the following issue would be appreciated.

Installed a new 6800 XT this morning, no problems physically installing it, booted up first time and all seemed well. I changed from another AMD card so I opened up Radeon software and updated to the newest drivers. Ran a game to give it a test, it worked perfectly for about 10minutes and then both monitors go black like they've been disconnected, and the system restarts itself. Signed back into windows, the background start up programs all load up, but as soon I as I go to do anything (even open Firefox) it Black screens and restarts again. Left it for a while, reseated the card, checked the cables were secure, tried again. Let's me open up a benchmark program, again, 5-10mins and it crashes. I figure maybe it's heat somehow, so I leave it and go to work for 6 hours. Boot up when I'm home, same problem straight away.
I put my old GPU back in, boots fine. Reinstalled the old drivers, tried again, still crashes. I changed the PSU the Sunday just gone, anticipating needing a higher wattage to support the new card, so I don't think it's that (changed to a corsair RM 750x, the recommended for the 6800xt is 750).

The card was sealed in the box, but it was a resold card that I brought privately. Which means per Sapphires policy, I don't have the warranty (yeah, I'm kicking myself now trust me). Seeing as it did work and still is able to boot up to at least the windows home screen,
I'm holding out hope that it's not just a dead card. Thank you in advance to anyone with suggestions.

MSi B550M
Ryzen 7 3700x
Sapphire Nitro+ 6800xt
Corsair 32gb DDR4
Corsair RM 750x