Double check the kickdown band with the pan off so you can grab the lever and move it by hand. It should move about 1/8th inch at the tip, mabey a fraction more or less depending on the ratio but 1/8th is about perfect. If you are positive it is good then pull the VB and pull the front band apply piston out and check the sealing rings. You can also check for proper functioning with air when the VB is removed and the piston and band all in place. Last thing I discovered after going through all this on a 904 doing the exact thing you describe, I had been useing ATF+4 and after about 3 fills and fooling with everything in the kickdown band apply circuits and no luck it was getting expensive and I was out of ideas. Finally one of the local tranny gurus here said the new fluid had way less friction makeing ability to give the newer trans smoother shifts. He recomended I refill it with typy F and I did and the problem was 100% gone. I have pretty much always used type F in old mopars and I will always use it. It made all the shifts better.