Not really sure if you're interested in my suggestion since this was posted a few months ago but since I haven't seen any one mention this in any of the posts about asrock p4v88. Check the capacitors on the motherboard (little battery looking things) if they are bulging or leaking then that's your problem. This board is known to have cheap faulty tachion (sp?) capacitors on them.
I had the same problems as you and after a week of endless troubleshooting I came across a website that explained this. I checked the caps and sure enough their tops where bulging and one had already begun to leak. When this happens it pretty much takes out the power controls on the mb (which is what happened to mine)
Hope this helps.