There are no error logs in the event viewer, it happens more often when I first turn the PC on, around 10-15 seconds. Or can happen at any time while windows is running. Only once has it not turned off, but the monitor has lost display. Changing DVI ports made no difference, and the kb/mouse still appeared to work.
Any ideas? The PC is a home built, approx 4 years old, with one CPU replacement in that time.