Well, it's idling at 35°C. I opened the sucker up, and... man. The dust. It was obscene. It was like I took the thing to the moon and threw it in a pile of the stuff. Blew and cleaned the hell out of it, and couldn't believe it. It was insane.
Anyway, I think that was all it was. I can't believe it was that simple (or that there was that much dust in there... I checked it not that long ago), but it's idling at 35°C now. I'll stress test it with the game and see what gives, but hopefully this is fixed now.
Thanks for the advice, everybody!
EDIT - Well, 20 minutes of gametime and it's gone up to 50°C. Seems like it'll be okay.