Computer Shuts down randomly with no error messages

Typically when under some load my computer will just shut down like the power was turned off. I have been having these problems for about a month but they have become more frequent. After it has shut down if I turn it on immediately it will usually shut down early in the boot process. It has also shut down when running off of an Ubuntu boot CD so I don't believe that it is a hard drive or software issue. The graphics card heat sink feels very hot after shutdowns and the cpu heat sink does not. Recently after it was built I checked temps and the cpu was running cool at less than 40 and the gpu was up over 70.

I have a machine with:
Zalman ZM750-HP Power Supply
Cooler Master Centurion Case
I7-860 Processor w/ stock cooling
Gigabyte H55M-USB3 Motherboard
EVGA 9800 GT Single Slot graphics card
2x2GB A-Data DDR3 1333 Memory
Vista Home Premium 32 Bit
WD Black 320GB Hard Drive (primary)
WD Black 1TB Hard Drive (secondary)
Sony DVD Burner

What are you checking temps with?
HW monitor from CPU ID would be my recommendation. Most common causes are GPU overheating and RAM errors.

Have you Memtested the RAM 8 passes min?
Are the RAM voltage and timings manually set in BIOS to manufacturer specifications?

Is there any OC? Is the OC Prime 95 tested as stable? Have your tried removing the OC and testing?

Are drivers up to date?

Don't really like that PSU choice, but I'd check the above stuff first.