Whats the power supply in this pc? brand name, model number, and watt rating. should be a big sticker on it someplace.
Quad core cpu means it will need to be a respectable psu. if you install nvidia's Ntune utility, you can view your video cards GPU temperature. Leave the sensor running, then launch and run a game that causes the issues with graphics, until you see the problem. then exit out the game and check the graph to see what the temperature hit. anything over 75C really is pushing it, even tho those cards are supposed to be able to run at 105C with no problems. Better case ventilation, possibly a better PSU, or replacement video card if that one is defective. One of these will be the solution. Overheating is my first suspect, followed by inadequate PSU.