Q6600 Stock -- Prime95 Error

[loldoctor]

I recently built a q6600 system -- specs below:
Foxconn P9657AB mobo
OCZ Reaper PC2-6400 RAM
Asus 4850
Thermaltake 500W PSU

Anyway, the problem I have is that even at stock, Prime 95 errors out as soon as I start Assassin's Creed at the same time. I tried OCCT, and that errors out as soon as I start Assassin's Creed also. Otherwise, the system is stable in everything I've tried (3dmark06, Crysis, Mass Effect, Prime95 w/out playing games)

Any ideas? Do I have a defective chip? Should I RMA it?

 

[DSF]

What voltage/speed/timings is your RAM being run at?

 

[loldoctor]

RAM is @800 Mhz asynchronous, 2.1 V 4-4-4-15

I think that's stock timings.