1983 Cadillac Deville Car is going dead in park

Engine Mechanical problem
1983 Cadillac Deville V8 Two Wheel Drive Automatic

I recently got new wires, plugs, rotor cap etc on my 83 Coupe. When I got my car back, I noticed that when you put it in drive or reverse it goes dead. I took it back to the shop and the assured me that it was nothing they did. Over the pat week, it has gotten worse. To the point that when you try to accelerate it smothers back. Once it catches up it runs smooth. But from a complete stop or even slowing down, when you go to accelerate it smothers back like it will cut off unless you let off the gas and kind of pump it. Now sitting idle in park the engine will slow down gradually and then cut off. My uncle was following me and saw black smoke coming out of my tail pipe when it was smothering back. What could be the problem?