Try using the DVI port of your monitor with a different video card and see if the same issue persists. If you were also using it on a different computer and not just plugging a different video card into your computer, it's likely the monitor that's at fault. If you tested the DVI cable and it works fine, then it's definitely the monitor.
Try using the video card with a different monitor using the DVI port. If the problem persists on a different monitor with the same video card then it's likely your video card has a bad DVI port or it could be a bad DVI cable.
Try using a different DVI cable. If this fixes the problem then it was obviously a bad monitor cable.