(WoW) Crashes in RaidsFollow

I have a PC problem, any help appreciated:

1. Only happens in WoW (Fallout 3 is fine on max settings).
2. Only happens in raids.
3. Usually happens when a bunch of **** is going on, adds, bosses, aoe.

Either:
Screen goes black (no cursor) and sound output sounds like a skipping CD.

Or
Screen completely freezes on one frame, with regular large pixelation, same sound skip.

Takes a reboot to get it working again. GFX card showing 40 degrees C. This is a self-build; if there's anything I may have forgotten to do, or any likely culprit, please let me know.
Dual Core E8400, Radeon 4870 GFX card, 2 Gig branded RAM.