We've had this problem at work a few times. More often, it's tied to subquality monitor cabling, bad video drivers (especially with Intel's really crappy "video" cards), and monitors that are dying.
If you're using an Analog video cable (blue connectors on each end) then swap it out for a digital (white) or vice versa. We noticed it most on the digital connections and moving to analog fixed it, but it's worth mentioning if you can try it either way.
Next, be sure your video drivers are up to date, and lastly, try testing with another monitor to isolate your current monitor, if able.