904 Shifting problem

Sticking yes i agree 100% but....... Why ?
Did you flush out the cooler and lines before your install ?
It might be wise just to pull the pan whilst your at it and just have a peek to see if anythings going wrong.
If it worked fine for how ever long ? Then all of a sudden it starts to hang up you might have something breaking up.
Better to have a peek than go through all the work of pulling the tail housing and find that its internal, And besides your under there anyway.

Just my opinion.... GL hope it works out for you.

PS: If theres no metal contamination evident in the pan i would suggest maybe try a cooler line inline filter and a Dacron trans filter. ( better flow with the screen style though ). Put a bit of a flare on the steel line for that inline i've seen them blow off with just clamps without any flare.