Hey Steve,

If the shifts are normally fine and the problem comes and goes, vacuum modulator is my first thought. Meaning check for vacuum leaks and also pull the vacuum tube from the modulator and check for fluid. If its pulling fluid then the modulator is most likely shot.
If the kickdown switch were causing the problem, I don't think it would cause it to stick in neutral. It could be delaying the upshift but I don't see how it would cause it not to engage into gear.

check out the modulator and look around the engine back for cracked vacuum connections. If you have a mighty vac, pull vacuum on the system from the modulator vacuum tube and see if it holds vacuum or if it leaks down.

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