I'm just guessing, but when you set the car in "drive" this will engage the clutch in the tranny, by means of oil pressure from the oil pump. the engine will then drop some in RPM. the idle speed on my 560 is also increasing and decreasing, I've read somewhere that this is normal, when the air condition compressor is engaging or disengaging by means of the magnetic clutch inside the compressor.

Mercedes560sel
NORWAY