I see that no one has responded in four weeks to your post, if you are still experiencing the problem I feel your pain.
There are only a couple suggestions that I can make, first you didn't make any mention of what hard drive you are using and I've on occasion seen hard drives that are in the very early stages of failure where the system will lock. So I would suggest you download and burn a bootable copy of Seagates Seatools and start with the short test to see if there's a problem with the hard drive. If that doesn't turn up anything then I would next try and download and burn a copy of Memtest86+ (you can find both Seatools and Memtest86+ through Google) and run the memory test for at least three or four complete run's. I've been working on a problem system that uses an Asus M5A99FX Pro R2 and Memtest would not run on this motherboard, apparently it uses an unsupported chipset and I had to take the memory out and test it in an Intel system that Memtest did support so if Memtest won't work on your system you may need to do something similar.
Have you examined the system error log? Are there any warnings or errors that give a clue as to whats going on?