The outside temp. monitor on my car starts with an accurate reading in the morning and then the temp. quickly rises 7-8 degrees. When I drive the temp. goes down but rises again when I stop. This cycle repeats until I drive at a sustained speed and then the car holds the lower outside temps.

Normal???

 

I dont know about the E55 but on the CLK's the temp. sensor is located close to the radiator and engine. Obviously if you are sitting still for a long time the heat from the engine/radiator will effect the reading (higher) of the sensor. Once you start its more accurate because the heat is pushed away from the sensor.

 

This actually starts immediately - before the engine is warm.