Not sure which transmission you have.

From the ASTG manual Transmisson 722.3 or 4
Transmission slips during the 3-4 shirt.
1 Check the modulating pressure and if needed adjust

2 Exchange the shift valve housing

3 renew inner plates of clutch K2, recondition clutch according to condition.

Concerning number 2 it was way in the past, but some member had found broken springs in the shift valve housing.

I don't know if the symptoms are the same if you have a different model of transmission.

__________________
84 300D, 82 Volvo 244Gl Diesel