Hi James,
Either your monitor is failing or your video card is failing. Assuming that you have a desktop computer, first make sure the cable going from the monitor to the computer is snug at both ends and there are no bent pins. If the cable looks OK, connect the computer to a different monitor.
If the problem is gone, the original monitor needs to be replaced. If the problem persists, the video card needs to be replaced. If you have a laptop computer, you can't fix this yourself. Contact the laptop manufacturer's tech support.
I hope this helps.
Regards,
Lokesh