My 1999 E320 wagon was beginning to have an irregular idle, and I instructed the dealer to perform a fuel induction service at 92,000 miles. Although I had never performed the service before and thought of it as a gimmick, the performance and idle of the wagon is like new. Service from their website:

Fuel Induction Service
(Recommended every 30,000 miles) $135.25
Cleans injectors, valves, throttle body, plenum and combustion chamber. Install BG Fuel Induction Kit. Road test.