Hi
If it feels like the clutch going, as you know autos don't use clutches per say, then it could be one of the brakes inside the gearbox, which is a big job and kinda of expensive (depends if you can talk nicely to an auto specialist)
Most auto boxes have more than one set of brake and fluid cluthes within the box one or more of these could be warn (not uncommon on a high mileage vehicle)
This is NOT a job for the local mechanic as it takes specialist equipment and as far as I know you cannot buy the internal components only less it's running a Mercedes box.
Charlotte