Just wondering, but did you check to see if they really did pull all the r-12 out of there? Was the ac working before you changed it? How long had it been since youve cut the compressor on, before you put new freon in it?
When I first bought my car, if you turn the a/c on and pushed the clutch in it would die (very dangerous when coming to a stop) I believe this is why the guy was selling the car, but all i did was put a new coil, cap/rotor, wires, and plugs in it and its never done it since.
I thought that the compressor has a clutch in it that is activated when you turn on the a/c if not...oh well...but if it does it should turn freely by hand. I would check the compressor out because as someone else posted i believe that is the only thing connecting the a/c to the engine.