BTW I’ve done some experiments (I’m off for Easter so I’m bored and determined to figure this out). Enabling Vsync to limit the FPS made me much more stable (albeit the black screen came back once). I’m only using about 25% of the GPU now. So I did another experiment - I quickly stuck a spare 700W PSU straight into the graphics card (and shorted it to turn it on without rewiring the whole system). I then stuck everything back on 4K ultra and allowed the FPS to go up to about 200 again… and after about an hour I haven’t had one black screen of death or restart (it was averaging about once every 10 minutes earlier).
This is bizarre, as I’ve been playing Doom Eternal on 4K ultra today and I haven’t had any issues. Just whatever AOE2DE is doing is bringing out the worst in the PSU.
For reference, these other people have been suffering the same problems as me: