Either the transmission control module is faulty, or you have engine coolant mixed in with the transmission fluid. Both problems can cause the same symptoms you're having. Some early 203's, 209's, and 211's have defective radiators, which allow coolant into the ATF. Vehicles built prior to 9/03, and equipped with radiators made by Valeo are prime candidates for the contamination problem. MB issued a DTB in January of 2005 addressing this issue. With the pressure cap off, take a close look at the coolant in the expansion tank. In severe cases, you'll see traces of ATF floating around.