I have '68 A body 4 speed in my Mirada the last few years. Last summer it started popping out of forth occasionaly only on decelleration and has gradually gotten worse. It isn't quite so bad when the trans is warmed up though.
From what I've read this is likely caused by the center bore of bellhousing being out of alignment with the crank. (I didnt check it for run out at the time, just put it together) WHich may put the input shaft at a slight angle causing wear on the engagement teeth for 4th on the input shaft and/or 3-4 shift collar.
I plan to pull the gearbox and check and fix the bellhosuing alignment if necessary. My question is it likely I'll need parts in the gearbox too? input shaft etc? ANy other thoughts on what it could be? I checked the shift rods alignemnt and it seemed fine, I even tried adding a little more length on the 4th gear shift by taking some from 3rd. but it didn't make a difference that I could tell.
What I have seen when they are way off is the end of the output shaft wears where the needles ride, the needles, the input bearing, and if ran long enough the input gear and cluster gear polish up pretty good too. You will also see a heavy polish where the pilot bearing rides. A trans that is dialed in looks like new at the pilot bearing. It used to be a big problem with blowproof bells, and early hemi swaps.
