I own a 1989 420SEL. I have not had your problem in my current car but in past owned cars, I have. My suggestion: test out the Engine Coolant Temp. sensor located at the top front of the engine. I had exactly your problem in an '89 GM car. Driving down the Interstate, the car would, without cause, shift up and down. The temp. sensor controls a lot of things in fairly modern cars. Hope that helps.