97 max a/c problem

My 97 max has a serious a/c problem. I live in florida where it is 92 everyday with 75% humidity in the summer. When I turn my a/c on, it blows warm air and then after a few minutes, it will blow cool air, then a few minutes later it will blow warm air again. On occasion, when I turn the a/c on, it will only blow warm air. I have had it to the a/c shop twice and they told me that the a/c is full of refrigerant and there are no leaks. The a/c guys can't figure out what the problem is. Could the a/c be overcharged? Any assistance would be greatly appreciated!