motherboard failure due to bad caps is the most likely explanation - http://news.cnet.com/PCs-plagued-by-bad-capacitors/2100-1041_3-5942647.html
I've got the same problem, reinstalled windows, replaced psu, memory, swapped video cards - no diff. And i've noticed the large blue capacitors on my board have crowned, indiciating failure
What i need to know, and someone else has asked on this forum, is where can i get a replacement board for the d2660-a1 motherboard found in p2511. This uses a microbtx motherboard form, and these seem to way way less common than micro-atx. Any ideas here?