Could it help to disconnect your battery for a while, then reconnect, to make whatever controls the tranny to "forget" what it has learned and start over?
Years ago I had an up shifting problem at about 50k miles. I was told on this forum that it sounded like slipping. I talked to a a tranny shop manager and he wanted to quote a rebuild without even a test drive. All that turned out to be bad advice because my problem, went away by itself. Never had to spend a cent, and it is still good.
My theory is the tranny somehow "learned" something wrong about the time to shift, and it eventually learned something better, but maybe I could have speeded the process by disconnecting the battery. Maybe I'm wrong about all that, but it couldn't hurt to try.
Also, a vacuum leak under the hood can sometimes cause trannies to shift badly. I once heard of someone who purposefully disconnected a vacuum hose so the car shifted badly, then took it to a tranny shop and the shop wanted to rebuild it! It was a test to demonstrate that the shop was crooked. Anyway, check for a leak if you can.