Screen shows fragmented and repeated images of itself and then freezes.

My screen shows fragmented and repeated images of itself and then freezes and I can then only shut down/restart the computer by holding down the power button. When the computer is restarted everything works fine. This happens most often when working for extended periods in Photoshop or Capture One Pro or both. My screen also tends to flicker when scrolling through web pages like CNN etc. I've been told it is likely my graphics card. Any thoughts as to what the problem may be?