Try a different monitor. If you dont have another monitor maybe your TV has a VGA or HDMI input to connect your computer to the TV to see if it acts the same. If it acts the same then its artifacts and its most likely caused by a failing GPU ( video card ). If its integrated video that is doing this then you can add a video card. If its a video card, then replacement of the video card should fix this. If its the monitor then you will need to get a new monitor its either a burned screen from prolonged use without screen change or refresh issue. If the monitor is new and still under warranty send it back for replacement. If the computer is still under warranty use the warranty if available to save money. Lastly it can also be cause on the rare occasion by shared memory with a RAM stick that isnt quite right. But its usually the GPU or a Monitor refresh or burned screen issue.