Watch your video card temps and/or test the game with a different video card to see if you can recreate the results. I lean towards video card problem but it could possibly be a power issue, although I think that is less likely considering the way the monitor acts. It really could be a number of problems. If it were me I’d just use old video card for a while and see if it happens again.
Anytime my monitor has shutoff but it wasn’t a crash it has had to do with my video card.