Sometimes this may occur if a graphics card is overheating, try to find an application capable of monitoring it's temperatures such as GPU-Z or AIDA64. Check the cable used for visual output going to your monitor for any ware & tear, and be sure to check the ends of the cable to make sure there is no damaged or missing pins. Try adjusting the screen resolution and refresh rate. If possible, try using that monitor on a different computer or try using a different monitor on your computer to see if the problem persists.