First thing to check is the cable. Maybe toss a different one on there just to be sure. If this fails, try hooking it up to a different monitor and see if the problem persists. If this fails, try some driver rehab. Uninstall the current drivers, reboot, install drivers again, reboot. AND if this fails, it is time to start looking at a new graphics card, or send in your current one if it is still under warranty.
Good luck, I hope it all works out.