I'd definately check the card is seated properly, as F5ing has suggested.
You might also check the cable. GTX460's typically have 3 or 4 connectors (Depending on brand). 2 x DVI, HDMI, D-Sub. You could check cable is firmly connected at both ends, try a different connector on the video card, or even a different cable.
If none of that helps, you can run some diagnostic programs on your video card to see if it has any errors. Furmark is one such program which can be downloaded from FurMark: VGA Stress Test, Graphics Card and GPU Stability Test, Burn-in Test, OpenGL Benchmark and GPU Temperature | oZone3D.Net
In my experience, Mini-HDMI connectors exhibit this problem frequently because of the location of the socket on the card, which is almost hard up against the PC case. And most 460's have Mini-HDMI.