Almost certainly a fluid level, or fluid thickness issue.

Check and see if your fluid level is correct. By chance, are you using any additives like that Lucas Transmission stuff?

I had the exact same problem last winter because my trans fluid was low and too thick (because of the Lucas). Did a fluid change ad the problem was gone.

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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