1996 protege a/c head ache

i've read on this forum about very simular a/c problems to the problems i'm having. my a/c works fine during cooler conditions, but around 80 degrees and higher it blows hot air, i've recharged the freon, had it checked for leaks, had the a/c relay check, and so on. by doing some further research, i've heard something about the expansion valve which is located under the dash. i've been told that it could be a culprit. i usede to hear a funny noise under the dash a few years ago while the a/c was working fine. what exactly does the expansion valve do? any advice would be helpful