If it turns out not to be the flex disks, it could be sticking transmission bands.

This happens to me on occasion, either on initial shifting when acccelerating on the first drive on a cold/cool day, or on deceleration after the engine/tranny is warmed up after sustained driving.

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black