See here - for me it was due to graphics card maxing out stressing the Power Supply Unit. I’m managing to play again using two PSUs… (obviously not ideal)
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…