The electronic throttle assembly may suffer an electrical failure. It is often cheaper to send the valve to a service center for repair that to purchase a new part.Google+
Car Problem Reports
Mercedes-Benz 400E Throttle Valves May Experience Electrical Failure
Mercedes-Benz 400E Problem
Visitor, 1993 Mercedes-Benz 400E, 156,000 mi
Engine park/neutral idle up and down rpm erratic/cycles. had smoke check for vacumn leakes. replace several vacumn lines previous. EGR valve and control relays can add to ,My mechanic got it smoothed out best he can without going down into electronic throttle control/cruise cntrl. Says drive it as is and save the the money as part assy. is big bucks. You can get it from Beckman's. I put in fuel injection harness too. wires were toasted and bare. much better after. I love my car, get 24 mpg at 70 to 85 mph.