When i go in-game to almost any game that has been made since 2003 I'll be ingame with absolutly great performance no slow downs or anything but about after 6 to 20 minutes the game with freeze for a about 20 seconds then resume with unwanted artifacts all over the screen and the textures messed up, and it's not the game files because this happens with almost everygame i play, Dark Messiah, MOH Airborne, CS:S, Etc. I really need help because i can't play my games because the game screws up so much with it on, i know it's not overheating because when it happens in game i go look at my temp and it will only be 40 to 45 degrees C, i don't know what to do i've tried so much to help it by, installing new drivers, newest physx version, etc. The Tom's Hardware community won't let me down i know. Specs
What about the power supply? Is it a good one, 450W is enough to run GT240. Did you buy it 2nd hand? I know I got 2 lemons from ebay. What other things are in your computer? To diagnose strip the computer to a bare minimum to function and then add piece by piece to figure out whats causing the problem.
If its under warrenty send it back.
If its keep happening all I can recommend is underclocking the card to see if it becomes stable.