I have read of ATI cards not increasing the RPMs of the fan when gaming and as you well know gaming will put a load on the card and thus increasing heat. I believe there is a tool to rectify the set fan speeds but off the top of my head I can't remember what it is.
It sounds like, at this point, a heat issue. Check your fan. Is it working? Also, download Everest Home free edition. It is no longer supported but it can still be found and check out your video card temperatures.
I would also post this issue under the Video forums here because this is more leaning toward your video card and temps and not really a device driver per se.