My experience confirms Benzmac's advice. The same thing happened to my wife in our 91 300TE, the shop thought it may have been the B2 piston. Ended up tearing the hole tranny down because the rear band actuator had a fatigue failure in the casting. I had all new parts used in the rebuild, but since your's was rebuilt 60 k ago, I don't know what to tell you. If you have the same failure, the synthetic oil wouldn't have made a difference. My failure was more than likely a casting defect.
I talked to MBUSA about some help and they laughed at me because the car had 150 k miles on it at the time. I can see their point, how many other brands would even be getting a call about something like this at this mileage?