i am not presuming to know more than MB Doc, however, in my experience and from several of the local 16v owners that i have spoken to, this is a common problem with this car. I myself have had this happen. It would appear that the root cause of the problem is the CIS system itself.
The Bosch CIS system is pretty good provided it is strictly the mechanical version. The KE system installed in the 16v is electronically controlled and as a result can be very difficult to set up properly. Two of the mb techs i deal with here are shop foremen in mb dealerships and both have owned 16vs. one still does and both of them say that this is normal behaviour.
one thing that seems to work in bringing the idle back to normal when it sticks is to lightly blip the throttle.
i hope this helps.
'94 W124.036 249/040 leder; 8.25x17 EvoIIs
'93 W124.036 199/040 leder; 8.25x17 EvoIIs, up in flames...LITERALLY!
'93 W124.036 481/040 leder; euro delivery; 8.25x17 EvoIIs
'88 R107.048 441/409 leder; Euro lights
'87 W201.034 199/040 leder; Euro lights; EvoII brakes; 8x16 EvoIs - soon: 500E rear brakes
'70 R113.044 050/526; factory alloys; Euro lights