You can partially diagnose the problem by disconnecting the shift control cable at the throttle wheel. Disconnect it entirely and take the car for a quick spin around the block or the neighborhood. Note that you won't have kickdown capabilities so drive it normally--no abuse.

If that corrects the shift point, then the shift control cable is problematic and needs replacement. If not, it's an internal problem with the transmission.

The shift will seem abrupt at higher RPM under partial throttle. The transmission is designed to shift smoothly at higher RPM under heavier throttle. The light throttle causes a jolt when shifting up.

Don't forget to reconnect the shift control cable to the throttle wheel. It's not very good for the trans to drive without it.
--
Chris Herbst
1992 745, 68k

And others:
93 944, 150k
90 245, 110k
88 744, 160k
87 245, 185k