1997 Pontiac Grand Prix What is my problem

The a/c doesn't work. I have charged it, myself, and the pressure guage reads the correct amout. It quit working on one of the hottest days of the year, while driving. The engine would get almost to the point of over heating if sitting idle in traffic, this only happens if the outside temp is 80 and up. The compressor clutch works and stays engaged when the a/c is turned on. I flushed the radiator 8 months ago as well, I cannot remember if that was before or after the a/c stopped, I think it was before.