|i'll bet your GPU is overheating, especially since the problem happens after periods of gaming. my 9800GX2 overheats sometimes and when it locks up, the audio will often rapidly repeat the half second or so of sound it was playing when the lockup occurred. very annoying.|
laptop graphics are even more likely to overheat due to the tiny amount of space and poor air flow.
Things you can do:
a) use BIOS/motherboard sensors or 3rd party software to read the temperatures immediately after a lockup.
b) use a can of compressed air to blow out the laptop air vents. dust accumulation can dramatically reduce airflow and increase temperatures.