The old 3 speeds did commonly jump out of gear with the synchro system they used. Worn shift forks also contributed to this. Borg Warner and later the GMC Saginaw had a much better system. But if the rebuilder was experienced, I would think that he replaced the synchro as well as the shift forks. I would strongly suggest pursuing what Cletis suggested. Either you have a worn pilot bearing and it is not uncommon for there to be no pilot bearing. It is not very common, but too much runout on the clutch housing can cause any transmission to jump out of gear. We check the runout every time we remove a transmission. As far as splines wearing at an angle, that is not common in Fuller transmissions. I have been running Eaton/Fuller transmissions for 45 years. I usually have from 10 to 20 trucks. I have never seen splines wear at an angle. If it were common, I'm quite sure I would have seen it.
