Have you checked to make sure that the fan on the card is spinning and not full of dust? I know that you said you've cleaned the interior of the case, but some people forget to get the grime out of fans.
I have a Intel Celeron with windows vista and when i try to play a specific character it will crash my computer. But when i play any others it doesn't. I downloaded Curse for my characters but idk if that has anything to do with it. plz help