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.