Any problems with starting a warm engine? I would think a bad temperature sensor would make the mixture too rich for starting when the engine is warm, possibly flooding it. I can't recall if there is a fault code for a bad temperature sensor.
It is beginning to sound to me like the problem could be related to a throttle setting. I believe the throttle position sensor for the Motronic system is a potentiometer with a wiper travelling over a resistance element, and as such could have a dead spot. Without knowing what is inside I can't suggest a test, but the idea would be to see if there is a smooth continuous change in resistance as the throttle is opened. With your problem, I would expect to see the resistance jump at the throttle position where the surging is occurring.
Slowly opening and closing the throttle with an analog meter connected to the TPS output should show if there is a noisy pot in the throttle position sensor. Can't say which pins on the sensor you need to connect to.