almost sounds like a sticking governor, or something in the valvebody acting up, but i'm no transmission expert by any stretch of the imagination.
consider this as a bump in the hopes that someone like Cope or other transmission expert sees this and can give the correct diagnosis and cure.
as to shift kits, i have installed them in every transmission i use, including front wheel drive transmissions[transaxles] starting way back in the mid 1970's.
they seem to work out for me doing so, but i like shifts to be crisp and be able to feel them. not bang/screech hard in every instance, but i really dislike the "slide into gear" shifts of some transmissions.
just my personal preference and experiences over the many, many years i have installed shift kits.
your mileage will vary.