If it were a w126 I'd point to a leaking vac system being plugged to make the transmission shift smooth, without that tell tale 'clunk,' and hard shift. If there a system to bleed the vac mounted on the valve cover?

I'm sure the one of the PO's had a specific reason to plug that line. Nevertheless, you're hedoing the correct procedure to find out why that plug is in the line.

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84 300SD
85 380SE
83 528e
95 318ic