it's called a thermistor, big guy in the fuel injection world....it's an interesting little monkey this engine coolant temperature sensor... I replaced mine a year ago along with new plugs and my 1990 300TE has really run like a champ since....that was last September and it's still OK.
It was the plugs and the coolant sensor....it was like $75 out the door, the car has been fine since.....not to say that this little piece of car won't tee me off again shortly...I'm not saying that it's perfect....but I am starting to understand a Mercedes Benz (my 1st) after about a dozen BMWs and
possibly appreciating what they are all about....they (MB) are quite similar to them (BMW) yet different....weird.....anyway..I would change the coolant temp sensor first and go to a general tune up next, starting with a spark plug swap and procede from there...that's what I'd do..best of luck...cliff