New System Shuts Itself Down?

I've never seen anything like what I'm experiencing, here, and I'm hoping somebody here has an idea. Windows runs fine without any real issues, but after 15 minutes or so of gaming, the system shuts off. It doesn't hang or reboot, it shuts off. Hitting the power switch will boot it right back up and there's no evidence that there's a thermal problem. I'm also not overclocking at all (yet) as I just took this machine out of the box at 4:30 this afternoon.

Here's the goods:

IKONIK Zaria A20 Case w/ HUSH Ultra 680w PS
Intel E8500
Asus P5N-D (nForce 750i)
4 gb Corsair Value DDR2-800
1 x XFX 9800GTX+ (512 mb)
Samsung 500gb SATA2 HD
LG Optical drive
Onboard Realtek HD Audio
Vista x64 with SP1
Drivers: Latest as of today (12/26/08)

I've uninstalled a ASUS AI tools, as I thought they might be messing with my voltages or CPU frequency. Windows Event Viewer reports only that the reboot was unscheduled, not what caused it. I have disabled automatic reboots on a BSOD in the hope that I could get SOME error message, but nothing so far.

I tried stressing the GPU hoping to see signs of a temperature management problem, so I installed the demo of Badaboom so I could see Everest's temp monitor while it ran - 62 is as high as I can get it and it's at 60 at idle.

I've tested it with Bioshock and Left 4 Dead and while both games look and play great at 1280x1024 on this rig, the system shuts down around the 15 minute mark. I'm downloading 3DMark now hoping to induce a failure I can see.

Any idea what's going on here?!