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!