I had the same problem with my GTX480 it happened do be related to the power connectors.
the GTX 480 has two power connectors, one with 6 Pins and the other with 8 pins. Your power supply cables must have an additional connector on the graphics cable (6 Pin + 2 Pin) in order to supply the necessary power to the GTX 480.
My power supply (850 watt Corsair) does indeed have 4 such cables. For some reason I had it in my mind that the second cable was optional consequently I only had one plugged in. Plugging the second one in along with the additional 2 pin connector made it work perfectly.
Thank you very much.
If you connect both PCI-e power connectors in the mothrboard, (the sata and the classic power connector), and enable them in the bios power section, you will notice a huge performance increase.