Re: The most annoying freeze ever!
Pull out every expansion card not necessary for booting. Also unplug extra drives that aren't needed to boot. If that corrects the problem, add components back one by one until the problem comes back. Obviously the component you added just before the problem will be your problem component.
Also, check your fan and make sure that it is providing adequate cooling for the CPU, and examine the capacitors on the motherboard for signs that they're near the end of their lives. They should be smooth and flat on the top, if they are rounded off, it's generally a sign that it's time to replace them.