i don't suppose there is a voltage switch on the back of your PSU? if there is one, and it got switched to the wrong position, it could cause what you're describing.
dave said, "I have a xfx nforce 780i mother board on the mother board it seems to shut down after 7F which is POST error check it seems to go to FF for a second then shuts down FF is Boot. "
If the PSU were set wrong, he would not get the motherboard LED codes. "7F" means it successfully completed the POST.
My first guess is a thermal problem because of an improperly mounted heat sink. My second guess would be some kind of problem with PSU.