You can run your Computer without the card. Right?
Maybe try anyone other card in your PC. Just to check out whether it is a Mobo problem or GPU.
When I got my new Computer then me too faced the same. Then I contacted the guy who built it, he said that connect the wire here & there.... blah blah.
And that done the trick!
So I guess its something Cable-o-Matic problem
Try fitting the Monitor cable in other slots. Fit it in every Slot, and try.