Its either overheating (more AA = more work for your GPU, and the temps outside are quite high lately) or there is something in driver control panel. Unfortunately I dont have ATI, so dont know its control panel.
But what does it mean: AA 8x, AF 16x, MSAA. 8xAA is antialiasing, MSAA is antialiasing too. Did you meant you selected 8x MSAA (instead of other type of AA?).
Do you have any problems while playing other games, or doeas it happen only with SH4?
Maybe try different driver version (some newer betas)?
There are 12.7 catalysts available, with performance improvements for 7900 series.
By the way (personal question
), do you have sun halo with 8xAA enabled?
Another edit: you can monitor temps, fps, gpu usage with msi afterburner.