I have a 1986 300E,which after being driven for 30 to 40 mins., tends to act like it wants to stall(but it never seems to),it tacs at 10500rpms and gives off black smoke out of the exhaust.I've changed the fuel regulator,fuel pump,spark plugs,O2 sensor and the over voltage protection relay. It has cleared up awhole lot, but every now and then it acts up the same way but without the black smoke to the same degree. I'm being told to change the coolant temperature sensor. Does this sound right? I'll admit I'm no expert on this subject.