I haven't got one of those processors (I moved over to the 'dark side' from AMD a few weeks ago) but somewhere in the BIOS there may be a setting that tells the system to slow down/reboot when the CPU temperature gets to a certain level. Try resetting this value to be slightly higher and see what happens.
Also, I would check your cooling solution. In particular, your heat sink and case cooling fans. Are they working/pushing air in the right direction?
P.S. How many fans and what cooler are you using?