I had a 680i that ran fine for 2 years then it started doing everything you described. I could run Orthos with no problems, game with no problems, pass memtest with no problems. Then out of no where it would just crash, was driving me nuts. Sent the board to ASUS, they sent it back the same way. Sent it back again, they sent it back the same way. Finally it made it to 3rd level support and they found a short in the mem slots, it was the board. I would test with another PSU if you have one to rule that out before RMAing the board.