I had a jeep transmission(Warner gear T150 3-speed) that had enough wear on the face of the shift fork(where it rides in the slot to engage the synchronizer) to let it pop out of gear. I'd suppose to much wear on the other side of the fork would not pull it out fully. I assume wear of this type comes from operating with insufficient or very dirty gearoil. Only happening on the 2nd to reverse shift sounds like it may be the shift tower. Try another top if you can find one, as I have rebuilt several manual transmissions and never heard that a top was matched to the case.
