You don't state where you are located, but if you are living in a warmer part of the world, your computer maybe affected by the heat. I say this because you stated that you didn't have this problem before.
Have you cleaned your computer out recently? Removed the dust bunnies from the case and fans? Cleaned the heat pipe or fan on your CPU and video card. The old 2100 is a lower end card, but this is a bit extreme and sudden as though something has changed.