Its not the Mobo, youve got the best one on the market for this setup. What brand PSU is that? If I had to take a blind shot at this, im guessing its an unstable PSU. What about your memory? Brand and configuration. As Wusy was asking. What temps are you at?
Intel P4 550(3.4)@<font color=green>5Ghz</font color=green>
Asus P5AD2-E-Premium
Crucial Ballistix PC2 5300 2X256
TT 680W PSU