stalling in traffic

This is often a Crank Shaft Sensor, but check the codes first. Did your mechanic check for any codes? Do you have a Check Engine Light on? This is are essential steps. The Cat converters should not have any effect nor should the tumbler, unless it is loose. The thing you want to look for in the codes is, are there any injector codes or misfire codes or Crank Sensor codes? If there are Injector codes, and Misfire codes it could be a harness problem which is very common on a Mercedes. The Crank Sensor Code is usually just a defective Sensor.