Ok so you want to save her. Well thats great especially that your mechanically inclined. I'm not sure if you had mentioned changing the detent spring or not in the begining. Anyway it can be done buy you with a better feel if once you split her open and of course you might as well update the trans. 2nd gear mostly . Mine mext time its out or actually I bought a trans to send to Robinson industries and have them install their billet 4th gear and undercut them plus they will go thru and correct/tighten up the shim tolerances then you cand drop the shafts back in. But first since I now know you have changed the detent spring I'm wondering if you got the new dpring verses the original one?
I say this because lets say the old orignal spring had a extra wrap making it a little long to match the larger solid disc detent wheel and they gave you a shorter one it might be overtension is your problem and in the ride time you have its getting weaker. I would check that first because well you know the price difference between the spring and a rebuild is a lot. If the spring is over sprungznd weaker the poping out of gear will start rounding the gear dogs. Just my 4.235 cents today