What you're seeing is called video artifacting. The first thing you should check is weather or not your video card is overheating. Be sure it's cleared of dustbunnies, and then check the temps. If your card has temp sensors on it, you can read those temps using
GPU-Z. Visit the video card manufacturers web site and find out what the normal temps for your card should be.
If your video card checks out fine and is not overheating, the next thing you should be checking is your power supply and make sure your video card is getting enough power.