Computer unstable after new case/ cpu heatsink. (Wall-o-Text)

Last night my pc started shutting off and restarting while playing games. Last week I got my new HAF X 942 case in, swapped over my parts and was gaming happily with no problems. Couple of days ago my Noctua NHD14 came in and I plopped that bad boy in. Gaming went fine for 2 or so days. Now, my computer shuts off and restarts itself, sometimes it just turns off, anywhere from 30 seconds to 10 minutes while playing a game. Nothing is over heating, temps are under 35 degrees. I put all of my clocks to stock just to make sure that wasn't it. Reset the RAM, checked connections, reset the heatsink, and all the other troubleshooting things I've learned from 2 years of working at a computer repair shop. Memtest showed no errors; reinstalled/updated video drivers; no capacitors are loose/blown; no static discharged onto my components; even unplugged the butt-load of 230-140mm fans in my case to see if they were drawing too much power.

The strange thing is that I can run CPU/GPU stress tests and the computer just chugs along. I've run Crysis in-game benchmarks 5 times in a row, no problem. Even ran Heaven 2.1 about 4 times with no problems. Games however last anywhere up to 10 minutes like I said and then my computer restarts/shuts off.

I'm baffled. I don't think it's the GPU (5850 vapor-x), because I can run those benchmarks with no problem. I don't think its the CPU as far as temps go (E8400@3.6ghz under 40c at load). It almost seems as if my power supply is going out (600watt OCZ gamexstream), but that doesn't seem right because I can run those benchmarks/stress tests like I said earlier. Who knows, there might be a short somewhere, but I'm just grasping at straws now.