Make sure your power supply is putting out adequate voltage, to do this google "how to test a desktop power supply" or "how to test for a bad psu" there are many articals written that show you exactly how to test the psu that you have. Figure out which psu you have, in most cases the wiring is the same, you will wanna power on your pc and packprobe each power connector checking for a good ground, as well as each rail for the correct voltage under load. If say your yellow wire or 12v rail is only putting out 10v while your pc is running then your power supply is failing. A bad power supply can cause lots of weird issues such as shut downs, freezing, glitching, artifacts, and many other issues. If one or more of your components is not getting the proper voltage than this is what happends.
Your power supply is a good one, but sometimes you get a bad unit. I would check this first and go from there. If the psu is good then move to the next component.
If you have extra hardware laying around try it, swap out the video card, the ram, hard drives, etc.