If you're willing to try out the heat question again, what about underclocking everything for a while? Granted the game fps performance will suffer... If it happens often enough (say once a night), then perhaps trying this for a week will tell you what's up.

I'm thinking underclocking (if successful) may lead to two things: 1) You have a heat issue when at normal speed 2) Your RAM is a lot happier.

You'd have to go from there to see which it was. Or if that doesn't help at all, you may have narrowed some of the possibilities out. Just an idea...