System won't Post - Asus P6T deluxe

This system has been working fine for about a year and a half now.

Specs:
Asus P6T Deluxe V2 mobo
Intel Core I7 CPU 2.66Ghz OC'ed to 3.6Ghz
6GB Corsair Dominator Ram
Noctua NH-U12P Heatsinc (2 Fans)
Western Digital
7200 1TB HD
EVGA GeForce GTX 280 Vid Card
Corsair TX750W Power Supply
Windows 7 Home Premium 64-Bit

Now I had a video card problem. RMA'ed the card and got a new one. The new card didn't work (I guess they just sent a bad card or somehow sent my bad card back to me). RMA'ed that card. New one seems to be ok.

About 2 days later, I power on the system. The system goes into Post (I get 1 beep) but nothing comes up on the screen. I reset the system. No Post beeps. No video but the system appears to be on and working (all light on mobo are on, all fans are running, green light on video card is on).

I then shut down the system and restart. The system comes on and boots to the desktop. I then go online and browse the web. I would say not 2 minutes later, system still running, the video goes down and I get a black or blank screen.

I reset the system again and again no Post and the system won't boot. I shut the system down and restart again. This time it goes into Post and I get a message saying that the overclock failed and to press F1 to go into setup. I go into setup and redo all of my overclock settings. The system boots fine and I have been using it for a good hour or so now with no problems.

Anyone have any ideas on what might be going on here?