I'm betting it's the CPU overheating. Games generally use up all the CPU power in the system so it doesn't take very long for the CPU to heat up, and without sufficient cooling it'll crash very quickly. Try checking the CPU fan and heatsink. Make sure the heatsink is on tightly, and the fan is going at a good rate. Also might want to check for thermal paste, although most heatsinks come with some. My CPU can survive without the fan on idle, but absolutely needs it at full load.