rough idle when heat or a/c on
 

Hi first timer. I have a 91 Cadillac deville 4.9 motor. When I turn on heat or a/c the car runs rough and surges when I'm driving. I did a tune up. Changed spark plugs and wires cap and rotor,fuel filter. And did valve cover gaskits. It runs good if I don't have a/c or the heat on. But when I turn ether one on the car runs rough and surges. Any help would be great.

I talked to a Macanic and he said unplug the a/c compressor. Did that and the problem went away. Now I have to buy one. Should I buy a new one for $250 or used one for $90