Replace your PSU if that is happening. Havoc2k5 is saying if your PSU output is out of range by even a small amount it can cause instability to your system when stressed.
My guess is that you need to remount your CPU heatsync with some new paste and make ABSOLUTE sure that it's seated completely flush all the way around. I've seen alot of my customers mount those CPU's with only 2 or 3 posts pushed all the way in and the CPU's overheat. They don't overheast immediately, but when stressed the slow down then lockup, or cause errors.