This computer was build in July 2011 and the problem began in November. I updated the BIOS, which seemed to resolve the issue until it reappeared yesterday. The computer runs perfectly fine until anywhere from 5 minutes to several hours after boot the video will cut out as if the monitor was unplugged and it does not return. The audio and wireless connection occasionally go out, though when I had a disc burning and the video went out, the disk burned just fine. Afterward, when I restart the system (it responds to the power button and I can hear the log out and shut down chimes), the system will reboot without any video, though it will POST fine and I can hear the startup chime as well as use my USB keyboard to type and enter my password to log in, just no video. This repeats anywhere from 2-15 times until eventually the computer boots fine, and then 5 minutes to several hours later the problem reappears. As far as I can tell, all of this is random. It also happens in both XP and Win7, during full boot up, safe mode and even at the BIOS configuration screen.
Steps I have taken:
Re-seat the RAM, GPU and CPU twice; check all power connectors; check all voltages; update BIOS to latest stable and beta versions; update all drivers; fresh installs of XP and Win7; run CPU and GPU stress tests; tried new monitors and HDMI cables. None of this returned any results, there are no crash dumps or other events being logged in Windows, no soft/hardware changes have been made that could have brought this on. Curiously, the computer has a telling sign for when it will boot with video: whenever I turn it on and the CPU fan immediately kicks into its highest RPM for a few seconds before slowing to normal, it always boots fine with video; if I turn it on and the CPU fan doesn't start going as fast as it can for like 3 seconds, I know the video will not show up.
At this point I'm assuming it's hardware but I have no idea what so any help would be appreciated.
MoBo: Asus Crosshair IV Formula
CPU: AMD Phenom II X6
GPU: Sapphire Radeon 6950 2GB
RAM: 8GB Ripjaws
PSU: Antec EarthWatts 750w