The stock heatsink for an Intel Core 2 Duo E800 has been working perfectly for my rig for the last 6-7 months of running it. Over the 6-7 months I went from an idle 50-51 degrees celsius to a 64-65 degrees celsius. The most logical reason I came up with was that the heatsink was coming loose due to movement of the tower and I simply needed to reposition it.
So, I attempted to reposition it, and now it seems impossible to retain a good connection. I thought since I had taken the heatsink off a couple of times, the thermal paste probably wore off a bit. So I went down to the local parts store and picked up some Antec Silver 5 and cleaned the CPU/Heatsink and re-applied the new paste, with no effect. The CPU is still overheating. I've had the push pins down so tight that I couldn't even budge the heatsink, and from what I could tell by the matching texture created by the thermal paste on the CPU and heatsink, the connection seemed fine.
One more note, recently my hard drive failed for an unknown reason. It simply died while my computer was idle and I had to replace it. I still don't know why this happened, but I'm just throwing it out there.
I don't know if this is still a heat problem or possibly something else. Can a heatsink actually break? When I put my hand over the fan on top of the heatsink I don't really feel hot air coming out. That of course might signal a bad connection, but I'm at the point to where I'm willing to bet money my connection is pretty solid. Why all the sudden would a heatsink that's been working for so long just fail? It leaves me to think the CPU is overheating for another reason, but I have no way of telling.