2.) check with a dealer as to whether the torque converter is a "switch-pitch" design.
I was mulling this over today as I drove across the desert from LA, and realized I had omitted this.
Cadillac introduced a switch pitch torque converter for the 1966 model year ( I've had 3 of these) in which the vanes in the converter change position depending on rpm and load- can lead to a feeling of "spooling up".
If MB is using a similar design it can feel/sound like slipping when it is just reacting normally to load.
balancing power with economy
Mike