i don't know if this wold help at all but my TA with the 4L60 liked to do stuff like that as well
if i manually shifted from first into second anywhere between 4500 and 5000 it wouldn't shift. it would keep revving peg itself out and jerk itself into second and jolt forward..enough to catch back up with the car i was racing.
after that happened a few times..shift kit went in and it's all good now
not sure if that helps at all..but i tried