Random shutdown/failure to reboot...

Hey guys. Hopefully this is the proper section, if not I apologize. I recently built a new system, about a month and a half ago. Everything was working great until last evening. I was surfing the net and my PC shutdown cold for no apparent reason. I restarted and everything worked fine.
Before I went to bed I shut down the computer. I went to fire it up this morning and got no response when I hit the power button. I unplugged the power cord from PSU and plugged back in and it booted briefly into the OS then procceded to go into about four loops of booting into OS for a few seconds then restarting until it finally took and got me to this point. I haven't shut it off since and have experienced no problems in about three hours. I have restarted and it's worked fine everytime but I haven't shut down cold again to try and reboot.

In that three hours I've run OCCT stress tests on my CPU, GPU, and both seemed fine. Temps are all good on components and the case itself is quite cool as well. It's a new build and I have a clear sided case and can see no sings of dust buildup. I haven't run memtest as I'm inclined to think if the power button is non responsive it's either my motherboard or PSU. Just curious if you've run into this before and what you'd reccomend trying first.

System specs:
Windows Vista Home premium 64bit
Core 2 Duo E8400 @3.0Ghz stock cooling
XFX Geforce 9800GTX+ XXX version
4GB Corsair dual channel XMS2 @800Mhz
400GB WD Caviar SE HD
Asus P5QL-E Motherboard @1333 FSB

I also get no error beeps on successfully starting up or non successfully for that matter. Also receive no BSOD error messages on restarts.
Thanks for reading.

Edit* Forgot to mention my PSU is an OCZ StealthXStream @600Watts.

Thanks for the reply Shadow. Wasn't sure if it would be an OS problem as the power button didn't respond when it was shutdown, ie, nothing running. PSU fan is fine and probably not a heat issue as it was shutoff for a few hours before I tried to start it again. I went to work and my GF said it restarted twice while I was away and restarted ok but didn't shutdown cold like it did last night. Took it apart and cleaned and reseated everything, see if that helps.

Hey dokk. I actually use Asuslogic for defragging. Way faster and more reliable imo than the stock windows tool. Just for an update I noticed my HD LED is now on and it wasn't before. It plugs into the same plug that the reset/power plug go into as well so maybe that wasn't seated as well as it should/could have been. I just finished running two series of tests with Memtest with no errors. Ran a defrag and also a full chkdisk also with no errors. My fans also don't seem to be powering up and down like they used to sound like. At different times it almost sounded like they were winding down like they were losing power then ramping back up again, even under no load. Dunno, seems to be ok now. I'm going to do some gaming and put it under a load for an hour or so and see what happens. thanks for the replies guys.