Problems with random power offs

[Uthred]

I recently put together the machine below, but im experiencing random shut downs where the power drops completely on it.

The specs of the machine

MSI K9N SLI-2F
AMD Athlon 64 X2 5600+ 2.8GHz Socket AM2
Corsair TWIN2X 5400 DDR2, 2048MB
BFG GeForce 8800GTS OC2 320MB
Cooler Master Real Power 550W
Seagate 320 GB IDE HD
Maxtor 250GB IDE HD
Seagate 250 GB SATA HD
2 x Wester Digital Caviar 250GB SATA HD

I thought at first it may be overheating but I've been running MSI's system monitor for the last while and even while doing heavy processing the system temperature has been a fairly steady 33 degrees and the cpu at 42 degrees. The voltage across all three rails also seems to be fairly stable. I've also run memtest in case it was the ram and it returned no errors. I thought it might be the PSU but the PSU worked fine on the system below prior to upgrade.

*MSI K8N NEO4-FI
*AMD Athlon 64 X2 3800+ 2.0GHz Socket 939
*2x OCZ GOLD GX XTC DDR PC3200 1024MB
*Sapphire Radeon X1800GTO 256MB
*CoolerMaster RealPower 550w +3/+12/+12

IDE00 - Seagate Barracuda 320GB IDE
IDE01 - Maxtor Diamondmax 250GB IDE
IDE10 - NEC DVD Recorder
IDE11 - Maxtor Diamondmax 250GB IDE

SATA 1 - Western Digital Caviar 250GB SATA
SATA 2 - Western Digital Caviar 250GB SATA
SATA 3 - Seagate Barracuda 200GB SATA

The only real difference are (in terms of power consumption) I believe the Geforce 8800gts vs the ati x1800 and the faster cpu.

If anyone has any suggestions on what may be causing the problem or how to resolve it I would greatly appreciate it.

[fren]

Remove all the expansion cards installed over the pci slot except the video card but before doing these, uninstall and remove its driver in the add/remove and in the device manager one at a time and start your pc. If it's still shutting down at random times. Replace your motherboard. If it's still under warranty, just exchange it to the manufacturer or to the store where you bought it.

[Chaky]

You might need more power with all that, and don't believe every PSU declared power output. I also have 550 w PSU powering dozen of fans, 2 HDs, 1 optical, GeForce 7800 GT 512 M, 3 GHz AMD Athlon64, and when I run ASUS Probe (monitoring software), I see voltages up to 10% lower than usual. 12V line gave me a couple of alerts. I would imagine that if I added 4 more HDs that I would experience same problems you are experiencing.