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........