Who did the work? Is it r-12 or 134a ? If they changed out the compressor and filled it with 134a, they may have overfilled it. When I installed A/C on my car it would almost stall between shifts with the A/C on. I lowered the pressure and it runs a lot better. Mine also shakes alittle with the A/C both on and off at stops (red lights etc, )-for you though I would check to see that everything is tighened back down. If it's overfilled it will probably shake too.