Computer worked fine for months, now lags 5 minutes into a game!

I have onboard graphics, and when I use a temperature monitoring program it says the gpu goes to 200+ degrees Fahrenheit. I'm not sure about the temperatures before this happened, but now whenever I play a game like Warcraft or Portal the framerate and sound becomes so choppy and unplayable. Could this be some sort of virus? Everything I have run has not shown any sort of virus. Thanks in advance!