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