System resets / locks up

Hi guys,

Can anyone tell me what could cause a PC to just reset itself. Then after the intial reset a bios lockup until it's completely powered down.

Sometimes locks up in Bios Screen, sometimes windows startup logo.

Ram test run for 72 hours with no errors, checkdisk run with no errors.

I can discount PSU and Graphics card as both have been replaced and it still happens.

Brand new mobo, PSU and RAM that are all listed as compatible. The only thing I am working with with that is not brand new is my Sata Western Digital Hard drive.... Could a hard drive cause lockups even in BIOS???

The only other thing it could be is a memory leak but how the hell am I going to find the route cause of that lol?

I did alter the voltage of my ram to the recommended settings in BIOS but to no avail, oh and BIOS has been updated to latest rom.

Thanks...

 

After a succesful shut down and being left for a considerable amount of time sometimes when powering it on it also locks up in BIOS.

The CPU tempature ranges from between 19 degrees and 35 degrees which seems fine.