I was just guessing.

The thought process went something like this. While your using basic OS programs your not really working the graphics card very hard, thus it's not likely to heat up, and the heat sink alone could cool it. But when your using the openGL in the poser preview, the hardware is working harder, thus your gpu is heating up and heat sink with fan should cool it. It sounds like the driver issue could be interfering with the cooling process (again just a guess). 

But I guess if after you close poser and all else works fine right away, perhaps I was barking up the wrong tree.
When you say P8 is stuck on the screen is this after you closed in the task manager? LIke the screen is not refreshing?