A/C swittch must be turned off then on to keep air cold

What is your question?

I have a 1992 Subaru Loyale, great running car, but with an issue that is puzzling me. My A/C works ice cold, but then after a while the air starts to get warm at which point I have to turn it off then back on, which then starts the cold air flowing again. It is very consistent in doing this, but I am afraid the turning on and off to maintain coolness will eventually hurt something. Can anyone give me some insight as to what is causing this? Thanks!