Ive had this problem and it will probably be your graphics card overheating.

Try taking the side off the case and point a fan or hairdryer (cold air) under the vid card close to its heatsink/fan. (also make sure the heatsink/fan is not clogged with dust e.t.c.)

Run the game and see if it crashes.

If it doesn't you can either get a fan card - http://www.pcabusers.com/reviews/vantec/fancard/p1.html

To keep it cool or change the fan/hetsink on the card (not advisable if its still under warranty).

Adding more case fans may help but i would go for the fan card.