Well my PC also used to get freezed up durinf gaming withing 15-20 secs.Sometimes I even lost the display , otherwise doing anything else was fine.Turns out it was my GPU which was faulty.Try using ur GPU in a frnd PC .
I agree with Calinkula, It sounds like it may be your power supply. I say this because when you are opening up a game or doing something that is intensive on your graphics card, it puts more load onto your psu to power the gpu. If this is the case, I'd change your psu right away to avaid potental damage to your mobo/graphics card. I learned the hard way about this btw, just this past week. Good luck!