Random Reboots! Help!

Hey all,

Been trying to figure this one out. My custom-built computer decides to reboot randomly, normally crashing on the mobo's splash screen thereafter. I know that it isn't overheating because I've been monitoring the temperature with RealTemp; since the computer immediately restarts, I've ruled out overheating. Temps are in the 30s-40s. I believe it has something to do with voltage. I've listed what I believe are the relevant specs below, but I am providing the correct amount of voltage as listed by the RAM manufacturer: 2.10v (http://microcenter.com/single_product_results.phtml?product_id=0301293). Other voltage settings I have left at the stock setting.

It's frustrating since it'll reboot when it's sitting idle, doing graphic design, playing Left4Dead -- whenever. Normally does it about 10-20 minutes after bootup. I believe it has something to do with voltage. Any ideas? Specs below:

Gigabyte GA-EP45-UD3P Motherboard
Intel Core 2 Quad 9400
OCZ Technology Gold Edition 4GB DDR2-1066 (PC-8500)
Corsair HX620W PSU
ATI Radeon 4870

CMOS Settings~
CPU Clock Ratio: 8x
CPU Frequency: 2.66GHz (333x8)
Memory Frequency: 800 Mhz
Timing: 5x6x6x18
CPU Vcore: 1.21V
CPU Term: 1.2V
CPU PLL: 1.5V
CPU Reference: 0.76V
DRAM Voltage: 2.10V
DRAM Term: 0.9V

Please feel free to ask for any other system specs! Help is much appreciated since I do graphic design at a profession and need a stable PC!

~Noah