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#11
If it's a desktop PC, swap the monitor cables around and see if the issue moves to the other monitor. If it moves to the other monitor its likely a video card or video cable issue. If it stays it's likely an issue with that monitor.
Is that monitor HDMI? If yes, do you have a cable box that uses HDMI to the TV? If yes again, you could connect the monitor to your cable box and see how it works there.