I have a 1985 500SEL that did somewhat of the same thing. Two things cleared it up:
1: Regapped the spark plugs- whoever did the presale tuneup just took them out of the box and stuck them in the car. Hmm.
2: Replaced a cracked vacum line. That really helped!
Mine was more a "knock" every 20 or 30 seconds when idling, and poor performance at speed.
[This message has been edited by jmann (edited 04-28-99).]