I'm not sure, but I think it is a hardware problem. For example, I haven't verified this, but I think I get less freezing when I unplug my usb keyboard and mouse. I've also noticed that one of my PCI cards tends to get unseated and this causes freezing, but I still haven't found a pattern.
Unseated or 'loose' PCI cards will definitely cause these kinds of problems... so its worth fixing that issue for sure. Secondly it maybe that your processor heatsink is not seated properly (or does not have enough paste between it and the heatsink)... we've seen that a few times which would go to the theory of 'over heating'.
Hope this helps you!
All the best