Well, the only answer to that would be that SH4, with GFX enhancements and your settings, is more demanding than the other games you are playing, whatever they are.
I will say that if SH4 is the only game doing this, that does lower the odds of overheating; but the symptoms, especially the time delay, are still symptoms of overheating.
Usuallly though, you will not suddenly just go black/green. As the card starts to heat, you will get artifacting and other strange glitches on the screen before it finally goes out.
Not sure what to say, just offering the only idea I could think of. Try putting in another game that you're certain is as graphically demanding. Crank all the settings ingame, and in your driver, to max, and let it run for 1/2 an hour, and see if it does it then too.
__________________
|