Have the engine up to operating temperature, get a piece of cardboard and wrap it into a tube and see if at idle you can stop the fan with the rolled up cardboard (Please be careful of your hands). If you can the clutch is bad and most likely causing overheating.
Waterpumps usually go bad and start leaking or the bearing goes bad. There is a thread that tells about the best types of impellers and how to optimize the distance of the plate behind it to get the ultimate efficiency which many times cures overheating problems. The replacement of a waterpump is fairly easy and can be accomplished with basic tools and knowledge.
I would read this thread:
http://forums.maxperformanceinc.com/...d.php?t=411256
to find out the idiosyncrasies of the Pontiac waterpumps and how to optimize them for best cooling. Hopefully this will get your Pontiac cooled down while teaching you some basics about how the pump functions best, good luck.