"I’ll adjust my throttle position ever so slightly and the transmission will suddenly kick back into second gear, revving the guts out of the little engine."
I remember this happening before I converted mine to manual. You could drive along just before the point of it kicking back a gear.
" If I pump the accelerator or put my foot down it will go back into 3rd " Now that is really weird! Usually I lifted off and it went back up a gear to 3rd, but then I'd lost so much speed I had to carefully to work it back up without it kicking down to 2nd again. Long slight uphills where I live.
" as soon as I release the throttle again it will kick back into 2nd again." Yes, that is the opposite of what should happen.
Isn't there a second cable on the throttle that does the kickdown at high throttle openings?? and a vacuum line from the inlet manifold to the box so it can sense inlet vacuum?? Its been so long since I had an auto now. If you disconnected them, can you drive it on the gearlever?? It should lack the kickdown then and just go up and down the gearbox depending on the rpm.