I did see a video on youtube on this fault .Thinking he changed the fuel relay Is this in a 260e? or 300e ? You wll find it at the rear of battery ,pull away the plastic membrain ,you will see another black box close to the ovp relay.It will be hard to remove as it as lugs on the side of the base that pop in to slots on the holder thats in there.This is if it like my 260e .Just another thing that can make these symptoms, look down on to the engine from the top with air box removed.Loook for this T shaped item with two hoses going to it and electrical connection on the rear. This regulates the air in to engine if its gummed up it will give fluctuation of engine speed.It is held in there with a plastic sadle just 2 bolts to remove it .Get it out and spray inside with carb cleaner ,shake it a little then tip it out. Do this few times.Refit , check for damage on the 2 hoses as you put them back on. Do let us know how it works out for you please..