Oliver, I don't think this has much to do with your transmission. Sounds like your engine is bogging down with the load placed on it by engaging the tranny. May require an idle speed adjustment. I would suggest a good diagnostic run at a Benz dealer, but from what you have been saying, that might get pricey. Could also be dirty injectors, some form of fuel starvation, plugs, dirty air filter, bad gas,...you get the picture. Could be many things. Check the plugs, check air filter, dump some injector cleaner in the gas. Take the car for a aggressive run in the country, in other words, go through the inexpensive stuff first.. Good luck...
2000 Dodge Grand Caravan SE