I have been overclocking for 20 years and this one really stumps me. I recently changed my motherboard and cpu from an overclocked q9550 at 3.8ghz to an Asus P8p67 mobo with i2500k at 4.7ghz. My computer is completely intel burn test stable and prime stable at 1.27v (except at night, let me explain). If I prime during the day, while I am web browsing, playing games, etc. no problems ever, though I never used it more than 5 hrs. When I go to sleep I set up prime to run for 8-9 hrs and every morning my computer is turned completely off. So I bumped to 1.28v two nights ago, and then 1.29v last night and same thing. My instincts told me it would be the PSU, but it is a new Antec 650 with a UPS battery backup and it never uses more than 400 watts and any minor power fluctuaction is taken care of by the UPS backup. It actually uses LESS power because the overclocked q9550 was at 1.4v and I also removed a floppy drive and bay controller. But I did put in a Corsair H50 instead of a TRUE cooler (which uses a little more power). The rest of the system is geforce 580, Corsair Vengeance 8GB, creative Fatality sound xfi, and a few case fans. Also to note is the i2500k does 4700mhz at 1.27v and very cool (intel burn test 60's on realtemp), but no matter what I do, won't be stable at 4800mhz, and I won't go over 1.3625v to try. Thinking it was the psu (I have a dual monitor setup), I put prime running on high priority on one monitor and played Need for Speed Hot pursuit on the other monitor to totally stress out the CPU, Geforce 580, and PSU. Guess what, this killer system had me playing for hours at a pegged 60fps while priming! What a chip! So it is NOT the PSU, ram passes memtest and windows memory diagnostics. So I am baffled. Do you think this is some weird prime only teething problem with the Asus P8p67 pro motherboard? Some bios setting wrong? I am running the memory at 1T? Or something with the Corsair H50 after 8 hours of prime? Also all my voltage readings are perfect, slightly above spec on 12v, 5v, and 3.3v lines. Any help would be appreciated, I am completely baffled. It's a great system, maybe I should just not worry about prime turning my computer off after many hours? I would just be happier to see an error in prime or even a frozen or blue screen in the morning!