Cavalier A/C system

Hi I have a 2002 cavalier with a 2.2L 4 cylinder engine. The problem that im having is with the a/c. I start the car and than start driving, once im driving I turn the a/c on and it blows warn air, than after a min or so it start to blow cold air, but when ever I stop at a red light/stop sign the engine start to idle up and down between 500 and 1000 rpm, and it starts to blow warm air again, than when the car starts to move again the idle goes back to normal and the a/c starts to blow cold air again. Also when I get on the freeway the a/c works fine while driving below 60 mph, and as I increase speed the a/c starts to blow warm air. This happens pretty much every day. I took the car to my mechanic and he told me all it needed was a little freon, so he charged it and for the first couple of days the car was working fine, but now it started to do the same thing. What could be wrong with my a/c?