Yet another boot up problem with a twist

lets start with that I have had a couple ''no boot'' problems from a low wattage PSU blow up to a generic 400w PSU's internal fuse tripping I guess cause it was a cheap PSU. Now I have a 500w that has been working great and is so quiet and produces little heat. But today I went to hit the power switch and nothing happens. The power light is on the MB. But here is the twist. I hit the switch on the PSU and the computer tries to boot up with the like 2 seconds it has (all fans and lights). Now if I hit the PSU switch back to on at just the right time it will boot up. But at other times it will just sit there with all the fans roaring and the lights on but doing nothing from there (I guess stuck in POST). I already said I have a 500w PSU, its switching and is ATX12V for my P4. The rest of the system is Intel D845HV MB, 256MB SDRAM, 1.6 GHz P4 400FSB, two optical drives, two HDD, 4XAGP Card, 4 case fans, and like 3 PCI cards on there. I think that covers it. Now, anybody ever seen this problem? Is it the PSU or MB? HELP!