Have you checked the CPU fan? If it is dragging (running slow or intermittent) it could create the overheat issue. Did you pull the CPU? if so, hopefully, you applied heatsink grease as is called for.
I've had my current computer for at least several years now, and over the years I have been tinkering around with it rather than purchasing a new tower. I've thrown in a new power supply unit, added a gigabyte of memory, and upgraded to an older NVIDIA graphics card a few years back. However, I have never upgraded my cooling fan.
The more upgrading I did, the more frequently it seemed to be overheating. I know that's what is happening because the computer suddenly and abruptly shuts down, and starts playing this two-tone siren-like beep from the CPU speaker. I have to turn off the switch at the back, wait for it to cool down, and then turn it back on.
This used to happen only very rarely while I was doing something intense, like 3D gaming or the like. But after I did the memory upgrade a few days ago, it's gotten to the point where I can't even play Aveyond II (a simple 2D roleplaying game) for more than fifteen minutes without it overheating.
I think it's time to upgrade. How do I find out what type of fan I have?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic