You must be from Europe, as there is no C180 sold in the U.S.
My 300E (3.2) had a similiar, crazy oscilating idle, and the mechanic speculated that it was one of the following:
1. a vacuum leak
2. mass airflow sensor
3. throttle actuator
After replacing the mass airflow sensor and then the throttle actuator, it was discovered that the real problem was the seal at the intake manifold. Replacing the intake manifold gasket solved the problem.
I was left with a $250 mass airflow sensor and a $1,000 throttle actuator that I didnt' need.
Which is why I now have a new mechanic.