I've experienced total system freeze ups recently when my system is under strain in a visually intensive game. I figured the issue was the GPU, and after some testing in Furmark it appeared that the GPU was indeed overheating and crashing.
However, I just got a new GPU tonight (HD 6950), and I still experienced a freeze up in a game. I decided to run a Prime95 test to get a better grasp on what's happening, and almost instantaneously my Worker #1 is failing, with the message "FATAL ERROR: Rounding was 0.49609375, expected less than 0.4." This is very odd, since my system has been totally stable for a couple of years now.
My system is
E8400 overclocked to 3.6 GHz
4GB OCZ Reaper PC2-6400
Radeon HD 6950
Asus P5N-D motherboard
650w Corsair PSU
My bios settings are
ram 4, 4, 4, 15, auto
Yes that's quite hot. Make sure the heat sink is free of dust, and definitely it can be worthwhile to reseat the heat sink with some new thermal paste.
Aside from that, it is possible for an OC to become unstable as time and electrons wear away at the circuitry. More than likely you just need to increase the voltage a tad - although at those temps I'd say instead you should drop the OC lower.