850 1996 TDI AW50-42 automatic transaxle hard shifting

Getting more and more acquainted with the "new" 850 TDI with the AW50-42
automatic transaxle, about 210.000 km driven, I realize that there is hard
shifting from gear 3 to 4 while the other shifts seem to happen much more
smoothly. There is a sort of rumbling through the car especially if I accelerate
heavily while the shift from 3 to 4 happens. I got the impression from some
written source that the torque converter lock-up may engage in any gear, my
impression is, that it engages only in 4th gear. Before buying the car I had it
checked, the OBDII did not contain any fault code from the gear box. According
to Volvo service plan the previous owner never had the oil changed. That is what
I will have done next. Could it have influence on shifting mechanics? Experts on
automatic gear boxes are rare here in central Europe, manually shifted gear
boxes are predominant.

Any idea about causes of hard shifting only from 3 to 4?

Regards

Viktor, Vienna, Austria, Europe

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personal e-mail vikwei at gmx dot at