Power supply is the next thing to check. The problem for you is that you probably don't possess a PSU tester. It's a bit of a gamble but you could pick up a new one for less than £20.
I'm suspecting hardware rather than software because you said it heppens before POST sometimes.
Another thing to try is remove all connected devices and boot it up. If it runs OK, connect them one at a time until it happens again.
-------------------- OK OK I can't go any faster! aaaaaaaaaaaagh!