A while back, my Dell laptop went on a streak of failures. Several of them happened to be your exact issue. First, it was the memory causing random BSoD's/freezes. Then, it was the hard drive. Lastly, it was the motherboard.
If your memory and hard drive passed the tests, then what I did to find out if the motherboard is the problem, was:
Take all unnecessary cards out, not including the hard drive, memory, and video card. If the freezing stops, it may be an optional card causing the problem. In this case, reinsert each hard one by one until it resumes freezing, and you've found the culprit.
If the freezing does not stop, we already know that the memory and hard drive are good; the only options left are: the Motherboard & Video Card.
I used
THIS to test the video card's stability. Of course, if it freezes during the test, this doesn't necessarily mean that the video card is the problem. Checking the error logs may shed some light on any errors that may have occurred during the test, however.
If you still can't determine if the video card is the culprit or not, try using a known good video card from another computer, or try borrowing a friend's.
If you do determine that the video card is not the issue, the only option left is the motherboard.
I realize I am ignoring the PSU, but this is because a bad PSU would normally cause crashes or reboots. The only case I can see that the PSU would cause
freezes is if it was not rated for your computer configuration - and this would have been apparent at the time the PC was purchased. However, this assumes the PC was not overclocked by you, and you did not recently upgrade the hardware.
