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#11
Another test you can try to eliminate the OS and hard drive as cause is run with a boot disk to see if you can replicate the problem, even with all HD unplugged. . A good one is Peppermint3 - Create Live CD/DVD/USB To Use For Emergency Backup - Windows 7 Forums
If the problem happens running a boot disk then you know for sure its hardware.
If not then I'd test the hard drive and work through the Troubleshooting Steps for Windows 7, or compare the perfect install in Clean Reinstall Windows 7 to what you have and perhaps skip to doing that now after wiping the HD with Diskpart Clean Command to clear all code that could possible interfere.