W202 Mercedes C 220 Mileage: 135558KM

My car or engine is shaking or idling, what i mean is i can feel the car is like coming to stop but it didn't for few seconds. The rpm come down when shift in gear and the engine is not smooth.

But when is in P or N everything is Normal.

I started to experience this after I brought my car to workshop for replacement of alternator and follow by engine wash.

Could it be the washing which spoil the wiring or anyting in the engine which causes this.