Idling

I have a 1994 pontiac Grand prix se..when stopping at a light or stop sign my idling speed drops about maybe 600 rpm..feel vibrating..I have had the tuned up completely,new IAC motor,fuel injection service done to it(Cleaning).It idles fine whild driving and at startup.Now when I had the fuel injection cleaned the car was idling way to high because it had needed a new IAC..maybe the IAC is dirty cause the injectors were not properly cleaned?..I have a new fuel filter on it as well..Had the car's idling speed adjusted by a mechanic on a computer..Could it be a sensor or what?..Any help is appreciated