Whenever I've had the thunking downshift it's been a result of a vacuum leak. The vacuum leak could be anywhere. In my case, it was in the climate control system and easy to check. When I shut off the climate controls, the thunk disappeared. So you could shut off the climate control, plug the door lock system leaving you with only the shut off system and transmission control system attached and see if that improved it. You could even disconnect the shut off system, plug it and isolate the transmission vacuum.
If you have a Mity Vac, you can simply check each system in turn to see where the vacuum leak is.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13