Automatic gears "fluctuation" in 2013 BT-50.

The gears between 3rd and top all lock the torque converter and what you are describing is what a split/failed one sounds like. I hope I am wrong. Talk to an auto transmission specialist.

Stephen

If you suspect that the problem is due to the PCM (Power Control Module) has learned some false information then it can be reset to factory specs by disconnecting the battery .

As these vehicles were built by Ford and some of their products require the vehicle to be 'shut down' for at least one hour, then disconnect the battery for at least an hour (may be convenient to leave over night) before reconnection of the battery , I would suggest that you not be in a hurry when doing any reset.

As this procedure is rather lengthy then most workshops will only use it as a last resort, if they even know about it, most other makers and the reboot procedure can be done in a matter of minutes.

It is normal for the vehicle to be a little 'funny' in its operation for a few km after any reboot, as it has to relearn some of its parameters.

Hope this helps.
Athol

Stephen,
Asking questions like that on a forum is a bit of a worry.
You will get some valuable information like that above but then will come a load of wild stab in the dark guesses from people that have no idea and will argue blindly that their guess is right.
Talk to specialists about it and more than one and unless they want to at least drive the car before quoting then find another.

This is not a stab in the dark, but a suggestion based on experience with my 2014 BT50.

What is the condition of your start battery? Is it on the way out and you're nursing it?

Reason I ask is Ranger/BT50s are notorious for being hard on the start battery with the way various computers and systems remain live or partially powered until the key is removed and the car locked. And the parker lights coming on if a door is not completely closed and other quirks.

If the start battery is weak and you get sluggish cranking followed by a start, this can lead to odd behaviours in the transmission. It did to mine with, IIRC, behaviour similar to yours.

I have no idea why this happens - you'd think once the engine has started and alternator voltage is up, that all systems would be normal, but apparently a very low voltage during cranking has a more than transitory effect.

When I replaced the battery the vehicle returned to normal.

Cheers

Stephen
In addition to what Athol and Frank have said, it may be worth having the pedal throttle control unit checked. You didn't mention if it goes away after a specific pedal position. AS some throttle units wear their carbon tracks and MAY lose contact ,ie, alter the info to the ECU to be different to what it should be for that pedal position. Usually happens all below fixed highway cruising speed position of throttle, below which, where there has been the majority of movement of it's internal carbon track wiper system. Similar to a traditional volume control/sort of. If that throttle position signal is erratic, ie, sometimes correct and sometimes not, the ECU and change computer section will definitely be confused. Some units are able to be opened to see and verify/detect for any wear of carbon tracks resistors inside. Slightly altering the path the wipers arc can fix the problem for a while. ie, bending wipers sideways 0.5mm or so. Usually there are three wiper arms and carbon tracks as back up for reliability reasons. Ingress of dust or moisture can also cause a problem.