probably the belt...i used to do a litte a/c work on the old R-12 days, before you needed a license to service cars. and you would have to run some water from a hose on the condensor coil(the one thats infront of the radiator) to keep the head pressure down so you could add freon to the system.. you had to watch it that you did not get the belt wet or the belt would slip and not turn the a/c compressor.. you probably either have a dirty or bad belt.. it takes alot of pressure to turn the compressor on a car..
or i would check the adjuster to see if the needle is in the center(if its on either side it means the belt is stretched and this would let it slip.... hope this helps.
