96 and up E320s have the electronic 5-speed tranny. Even a W124 E320 tranny will not work - MB made a different one specifically for the 400E/E420 to take the added power and torque.

I hope your drain & fill / filter change works to fix your problem. Does any one know about tranny flushing? It seems this might help to dislodge more gunk - I'm not sure if this is a myth.

Regards,
Troy
1995 E420 75k