On the right rear of the trans there is a wire hooked to a solenoid. Disconnect this wire and then drive the car and see if that helps. If it does you have a problem with the kick down switch under the gas pedel or some other electrical problem. If it helps let me know and I will try to help. If it doesn't, try to remove about a quart of trans fluid and replace it with a quart of 10w 30 motor oil. (this was recommended to me by the best transmission man I know) It fixed a buick I had before my 500SEL. If that works and the trans is working correctly then take it to someone that knows MB trans and have it serviced again making sure the Torque converter is also drained.
84 500 SEL (307,xxx miles)