Could be heat. If you don't already have one, try installing a slot cooler in the slot below your GeForce. It'll suck out all the hot air the heatsink/fan on the GeForce is blowing around. The heatsink/fan is useless if the hot air doesn't get removed from the area around the card.
Or, worse, it could be a power problem. Look at the I/O voltage in the BIOS, or better yet using Motherboard Monitor 4.16. For a GeForce, you want it significantly above 3.3V, as close to 3.5V as possible. When 3D gaming, the voltage can drop below the nominal 3.3V and the GeForce shuts down. This happen mostly in older mobos like yours. If this is the case, return it while you still can.
1.Old Antec Case, Asus PC-DL, 2 x 1.6 LV D1 Xeons @ 3 GHz, 2GB Corsair PC3200 @ 200 MHz, Chaintech 6800 128MB AGP
2. Old Crap Beige Case, Gigabyte GA-H55M-S2H, Core i3 530, 4GB CL8 OCZ DDR3-1600, eVGA GTX465
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