A lightbulb just flashed on over my head!
One additional possibility on your transmission shifting problem: Fluid incompatability with internal clutches/bands.
It just occured to me that you mentioned recently changing the fluid. Not insinuating that you did anything wrong but if your transmission has never been rebuilt and has the original clutches/bands then it requires the exclusive use of, I believe, Type F transmission fluid only. The wrong transmission fluid can definitely cause shifting problems.
Or, if at any point in the transmission's long history an improper transmission fluid was added the possibility is that insufficient lubrication has occured that over a long period of time could lead to problematic transmission operation you are now experiencing.
I wouldn't consider this possibility to be the most likely one but it is just another potential factor that could be looked into.
I'm just throwing any potential scenarios at you that may not otherwise be considered.
Good luck, again.
1968 F100 -360, 2BBL, C6, 3.25, ugly? yeah, she's ugly.
1997 Expedition -4.6L, 4WD, 14MPG, gutless? yeah, she's gutless.
2001 Kawasaki ZRX1200R, fast? yeah, she's fast.