I have been getting this problem playing tf2 lately.
I have an hd 4670, and watching msi afterburner/ccc I see a max of 55c, nothing more. That's not THAT hot. I've tried all of the latest versions of control center.
Sometimes it happens after 10 minutes, sometimes not for 45 minutes. I have a corsair 750w PSU, which I doubt is the problem but I suppose it's possible.
Edit: Just happened while I was in the first screen when connecting to a server, where you hit "continue" to choose a class. This is so annoying.