Random Shutdown when playing games

When I play a few games, like Spiderman: Web of Shadows, or during the cinematics in Battlefield 3, my computer speeds up and then shuts down. I purchased an external fan and games like Mass Effect 3 which would normally crash on my were fixed, but it doesn't affect Spiderman or BF3. Any Help?

Nvidia GeForce GT 525M
Win7 Ultimate 64 bit
6GB RAM
Intel Core i7-2630QM @2.0 GHZ (Turbo Boost to 2.9)
I'm not sure about the PSU

 

Download and install this...

http://majorgeeks.com/Speccy_d6358.html

Record your CPU and GPU temps while on idle, then fire up a game, record temps again after 3 or 4 minutes.

This should give you a good indication of whether you have a heat issue.
Since your installment of an extra fan helped, I would hazzard a guess that you are over heating.

Make sure your pc is free from dust, concentrating especially on the CPU heat sink and the GPU.

Post your results.

Oh, and welcome to the forums