02 AC problem

My 02 AC is giving me problems. It blows cold air sometimes,it wiil switch to warm air and I can;t get the cold on.I switch the controls on and off ,up and down on the temp and it will go back to cold. Lately it quit getting cold,I checked the AC system with gauges and the feon pressure was low .The compressor was cycling on and off.Filled the system to the proper pressure and the comp stayed on but still warm air inside the car. The compressor lines are cold. Now what ! Is there something wrong with the ducts not closing or opening properly? Hope someone can help,thanks.