You really won't know for sure if you have vacuum problems until you put a vacuum gauge on the line going to the transmission.

I had the same problem as you for several months until I got a vacuum gauge and had an accurate diagram of the car's vaccum system.

It ended up being three inter-related problems:

1) Vacuum hose completed disconnected at the transmission modulator.

2) Vaccum hoses incorrectly hooked up by someone working on the car before I owned it.

3) A restricted vaccum supply at the tee fitting off of the larger diameter vacuum hose running to the brake booster.

Even after solving the obvious problem #1, it wasn't until I was able to observe the vacuum readings on a gauge that I found and fixed problems #2 & #3.

A vacuum problem is the most likely thing to go wrong considering the usual robustness of the Mercedes transmissions - and it also happens to be the easiest thing to fix - the thing you want to know is working before you go deeper into other possible fixes.

Just my limited experience so far........

Ken300D