Calder, ok, most ATI/NVIDIA drivers support showing the temperature. What you might want to try is to see what the temperature is after such a crash.
These kind of problems are hard to diagnose, for me I from time to time experienced spontaneous reboots. These can be drivers, but also memory, PSU, heat (although I think that only caused crash to desktops for me with Star Wars: KotOR), and some other factors.
Can you try this:
http://oca.microsoft.com/en/windiag.asp or
http://www.memtest86.com/ or
http://www.memtest.org/ and see if you pass through the memory tests so we can at least rule that out.