I have a '97 S320 which runs like a turbine EXCEPT at idle. The idle speed is slow (looks like only about 600 rpm on the tach) and lumpy, but if brought up a few hundred rpm (either by the pedal or if the a/c comes on) it smoothes.

Is there an easy-to-locate idle adjustment?

On this car & most after 1982 there aren't any idle adjustments. All computer controlled. Some cars upto 1993 could change fuel mixture but the computer still had control of mixture. If your car has rough idle OR feels that way the MOTOR mounts may be failing. They are hydraulic.