Thanks!
Using that, it's reading my max CPU temp as 100C while playing Rocket League (a game it has never crashed during), about 80C when not running a game. I haven't had a chance to check during a game it has crashed on (Beat Saber, Borderlands 2) to see how hot it goes then.
I'm not clued up on these things, but that seems hot!
On Intel's page for my CPU (i7-8700K) it seems to be saying that's the maximum allowable temp ("T-junction"), so this does maybe suggest overheating is the reason for my woes. Can anyone more knowledgable confirm?
If so, I guess this has turned into a hardware question. Fans and AIO cooler pump all seem to be running fine, so why is it getting too hot?