Thermostat could be to blame (stuck open). Considering your car is a bit of a problem child (no offense intended) replacing the thermostat is an easy and cheap possible fix. The problem could be within the maze of temperature sensors and vacuum lines that make up a Mercedes climate system, but replacing the thermostat to help the engine maintain a constant temperature is not a bad idea.
Give it a shot- I think the part is like 40 bucks on FastLane.
1995 C36 AMG