AMD RX 6800 XT Taichi

The first thing I would do is swap in another PSU to test with. It sounds like yours
has degraded and is losing stability once the draw exceeds a certain amount.

If the GPU was not showing a display at all or was artifacting badly then I would
suspect that but that isn't the case.

What PSU are you using with the system and what are the rest of your specs£

If it is exhibiting the same behavior on multiple systems, RMA it. Try through the
retailer you purchased it from first, if that isn't possible contact ASRock through
their website.

While it could be something as simple as the cooler not having enough mounting
pressure or a bad thermal pad, it isn't worth risking your warranty to fix it
yourself, particularly if there is no guarantee you will succeed.

Your best bet is to check over the card, make sure the cooler isn't loose, check
for any excessive dust build up etc.

How long did it function correctly before the issue started£ This is important
because if it worked fine for days to weeks then suddenly stopped that is bad news.
However if it exhibited this behavior within a few days of installation it could
be a simple fix as I mentioned above, loose screws, thermal pad out of place due
to improper handling in shipping etc.

Examine it closely for anything that looks out of place.