I think Scott is close in his assessment - your problem probably lies in the governor plugs/shift valve/throttle valve valve train.

The fact that you get a clean 1-2 and 2-3 on a steady state throttle shift lets us know the L/R band is not hung up and likely the front band is ok too. Sounds like maybe the throttle pressure/kick down valve assembly is your culprit.

Not knowing what this kit has for parts and what it has you doing - I would return it back to stock and see if things get cleaned up.