reseat your ram, check how much you have. Are you mixing sticks?
I was having almost this exact same problem until I figured out that my computer was only recognizing like 2 gb of ram because I had different brands of ram in different slots (like an idiot). I fixed it by taking all but one of the same brand out, and reseating the remaining sticks.
(and yes i know you said you ruled out all hardware problems but it's worth a shot)
If that doesn't work, try running Prime95 for 24 hours (with no other apps open) and report temps and stuff back here.
By the way, you could reinstall windows. That has a pretty good chance of fixing it (and any other problems you might be having)