Mercedes-Benz: question ?? 420SEL

thanks for the input concerning the smoking problem. i'll be having bot
the modulator valve and valve guide-seals checked tommorrow (if it were
the valve guides wouldn't it be using oil.
today the starter smoked and i had to be towed in for repairs.
another question though concerning the cruise control. it appears that
the control circuit works and holds the engine rpm at around 2000 to
2200 rpm but it will not latch in mechanically and regulate the speed of
the car. scenario goes like this, at around 60 mph i set the cruise
switch, it does not give any indication of working so i shift to neutral
and the rpm is holding at 2000 rpm. i click the cruise to off and the
rpm drops back to 900 or so, pull it back in gear and resume speed, it
has wprked in the recent past and might be a vacuam leak but i don't
know how this system work exactly, any suggestions as to where to start.