I had similar behavior in mine with a broken K1 accumulator spring. When cold it felt like someone was running into the back of the car when it shifted from 2-3. BANG! Once it warmed up, it was acceptable, but still a bit on the firm side for my taste. My K1 accumulator spring was in pieces. Replacing it and fluid/filter change and it's nice and smooth.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)