There is a sevice information article about low idle on your model. The repair is to interupt the purple wire from the starter terminal 50 to the engine control unit. Take the wire coming from the starter and insulate its end. take the end going to the engine control unit and hook it to ground. This increases the idle by about 100-150rpms. Sounds like a possibility here.
Bosch Master, ASE Master, L1
33 years MB technician