Heat dissapation is the antagonist here, and somehow, your rig isn't getting cooled enough. Is it located in a desk cabinet with the door closed or is it on the floor or on top of the desk? I moved mine out of the tower cabinet on my desk for the very rasons you state here. The internal heat was way too high, even with high-end fans and heatsinks in place. Mine wasn't freezing or re-starting, but performance was suffering noticeably. Now that it's on the top of my desk, the problem has ceased.
Did you buy this rig from the store or is it something you bought from a previous owner? If it's a pre-owned rig, it may have some after-market tweaking on it.
Whatever the case may be, you need to get the heat controlled in the rig, or that CPU is history, if it isn't already. The high end temp for your CPU is 68-70 degrees celcius, so you're not critical yet, but you're awfully close. Here is a link that gives some specifics on voltages and temps, hope it helps. http://users.erols.com/chare/elec.htm