Every so often my trans will downshift from 4th to 3rd and will not go back up. This will happen at steady highway speed. Upon slowing down shifts fine gears 1 to 3. Shutting off the car fixes the issue. Trans now shifts perfect 1 to 4. Just starting to diag the issue, wondering is this has happened to anyone. Stuff I'm chasing initially will be gear selector and kickdown switch on the accel cable.

 

With no trans experience, the first question that comes to mind is what is this transmission's history of fluid changes?

 

Gear selector position sensor?