This was happening with my old PC's graphics card whenever I tried to play WoW. What is happening is that your graphics card is overheating to the point where it shuts itself down. Honestly, you are going to need a new graphics card as this will continue until it eventually dies, unless you can figure a way out to cool it down. Here are a few tips
1. buy some compressed air, from staples or officemax or a computer store, and remove the side panel of your computer and clean out all of the dust. This can make a huge difference in you GPU's temperature.
2. Undo the side panel and place a normal living room fan right next to it and allow it to run directly into your computer. While this is extremely "ghetto" as the kids would say it can help a little bit if you really can't buy a new card or heatsink. This is definitely not recommended, but a last case scenario.
I hate to be the bearer of bad news but you are most definitely going to need to replace your graphics card and or heat sink. So long as your card is freezing up likoe this the worse it will get until it eventually dies. Good luck with everything and please post more questions if you have them. I am happy to help!