Spanners, only just caught up with this thread. Did you say you were getting screen corruptions before freezes? Like some pixels just aside of the the mouse pointer?
If so, my guess is a memory corruption problem, either caused by bad drivers for the video card, or bad configuration between the video card and bios, or just a plain broken video card doing duff memory cycles.
If the problems go away with another brand of card installed, you know the answer