What could be the reason behind a harsh downshift between 4th and 3rd on my 1990 420SEL w/217k miles or so? All the other gears shift fine up and down.

It is possible to do a smooth downshift if you are barely touching the throttle and manually shift from 4-3 with the lever. If you're doing 60+mph and you plant your foot to the boards, it jerks the whole car like a shotgun has gone off! Kickdown from 3 to 2 is perfectly smooth at any speed, it's just 4 to 3...

Any ideas?

Cheers, Martin

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1990 420SEL 217k Miles
1975 Chevy K10 400ci SBC power!
1995 Jeep Cherokee 4.0 Limited
1997 Suzuki GSXR750 SRAD
1989 Kawasaki GT550G4