mondragor, what's funny is that our problems are really identical. Definitely someone to share the pain with
Myself too several times I thought I fixed the problem, only to find out a day later that it still crashed. Also, when I run heavy games (starcraft 2 at max settings on Win7; war3 via Wine on Linux), sometimes for several hours, it does not crash at all, mostly it crashes while browsing the net and moving the mouse on the browser... same as you too.
We have almost the same PSU (500 W in both case). If you exchange yours for another more powerful one, please tell me if it fixes your problem