You're trying to game on a 9400GT, really? Anyway, it could be overheating. Make sure it doesn't have a build up of dust on the heatsink and that the fan is blowing if it has one.
Thanks for your reply. I tried all these and the Fan is working well. It happens only when I try to play game, otherwise it's ok.My nvida drive is Version 266.58. Can you plz help me