Most likely it has something to do with memory of some sort. Whether it is actually your RAM or the memory on your video card.
Also heat may be a contributer here since this happens when you play games you might overheat the VRAM and cause it to fault. Check the cables and fans around the card to determine if it is being cooled properly.
As far as the ram same as fowang said check to see if your system ram is faulty. Use MEMTEST86+ to check your RAM.
Post back with your findings!