First, something i should have asked in the first plaec, how long does it take to crash, if it crashes before you start playing then i will have to look into it, if it takes a while to crash then, it could be the northbridge overheating, to test this open the pc, and put your finger on the little heatsink(usually coloured gold) below the
cpu cooler, if it burns then that could be the problem, if not then read on:
try running a stress test software, like orthos:
techPowerUp! :: Download Orthos Stress Prime 2004
you will need winrar or similar to extract
Then download speedfan from here:
Download SpeedFan - Access temperature sensor in your computer
then run them both, while keeping an eye on the temps in speedfan, generally the highest is
cpu, if it increases suddenly when orthos starts testing, that is it, keep an eye on this and report with its highest, if it doesnt crash we can rule out a
cpu overheat problem
to test wether it is the gpu overheating, run atitool witht the 3d window open, and keep an eye on the temps that it reports, reply back with the highest once again, get atitool from here:
ATITool - Overclocking utility for ATI cards
run bot the tests for at least 20 minutes to get it to heat up, but longer is better