There are many possible variables even within the video card. You could try removing the video card and using onboard or a spare to see if it's the rest of the system first.
If it's not the system then you can try putting the video card in another system and trying it with the same monitor and cable. If it works then your video card might not agree with your system for some reason even if neither part is seems defective otherwise.
I assume the problem is the video card because generally any problem on the screen is a video card problem.