So many people with such a problem.
-When playing games, your computer cpu is used much more..
-Graphics cards are used much more..
-They create more heat..
-Some laptops and indeed computers can overheat..
-There is a failsafe.
-If a computer getsd over the pre-configured temperature, the computer restarts or shuts down.
-This is to avoid your cpu being damaged.
You can check if this is happening to you by downloading "coreTemp":
32Bit Windows http://www.alcpu.com/CoreTemp/CoreTemp32.zip
64Bit Windows http://www.alcpu.com/CoreTemp/CoreTemp64.zip
If the temperature is 90 degrees during your gameplay or infact if it is around 75 degrees whilst just using your computer... This is probably the issue you are coming across.
Buy a cooler for laptop:
Desktop Computer Solution:
Replace your Heat Sink and add more fans to your computer.. Probably best to take to a computer repair.. If you have no idea how to or look up on google how to. It is relatively simple, however sometimes you need to remove the whole motherboard to place the grip on heatsinks.