Computer reboots randomly, black screens

[NovaX81]

I just finished building a new rig, but I'm having some interesting issues; occasionally, almost randomly, the computer will simply reboot instantly, sometimes requiring multiple attempts to get back into windows (which I just installed, have basic apps installed).

Occasionally it will even do it when I enter the bios. Also, sometimes, it wont fully reboot, just go to a black screen with no video output, but the power remains on.

Memtest run in XP got no errors checking the RAM, and Catalyst Control center doesnt mark anything wrong with the videocard. Heres the system, maybe you can think of something I havent:

Gigabyte GA-P45-DQ6 mobo
4 GB Mushkin 1066 ram (2x 2GB), changed to stock settings in bios (5-5-5-15 @ 2.1v)
Intel Core 2 Duo E8400 @ stock
Sapphire Radeon HD 4870 X2 @ stock
ASUS Xonar DX2
WD 6400AAKS
Mushkin 800w modular PSU

any ideas? I think I've narrowed it down to either the RAM or HD, but can't be sure yet.

[Psychosocial]

^^Must be a PSU or a HDD fault.

[amrawtanshx]

^^
Correct.

[acewin]

PSU fault, lesser chances of new HDD giving the probs. Check all the connection again.

And check if u got any other HDD and keep it in to boot.

[realdan]

did u install the processor heat sink properly?

[teamonesolutions]

Hi,

Check your RAM & SMPS Only.

reg

Mahendran

[spindoc]

+1

PSU problem

[lywyre]

+1 -> PSU / RAM.

To be sure, press F8 while the system boots up and choose -> Disable automatic restart on system failure and proceed. When the system fails Windows will display a BSOD. Note down the error and type it here or google it .