For what it's worth, I had a problem with the car "jumping" out of gear - 2nd and 3rd only - while I was driving. It was intermittent, and would sometimes startle me when I wasn't expecting it. I would just depress the clutch and put it back in gear. I never really had a problem finding any of the gears like you're having, though, and they'd go right back into gear - for a while. I wasn't sure what was causing the problem and a few people I talked to blamed the vacuum shift system. Well, I had a bad bearing that was making noise and had to be replaced, so a friend (who knows much more mechanically than I do) helped me pull the tranny. Got a rebuild kit and had someone else do the rebuild work on it.
When we pulled the tranny and opened the cover, the first thing we noticed was that the speed yokes (there's one for reverse and 1st, and one for 2nd and 3rd) were badly worn, especially the 2nd and 3rd speed yoke. Where the machined opening should have been somewhere in the 1/4 inch specs, it had worn to about 3/4 + and that, we determined, was causing the jumping. That tranny gear just couldn't stay put where it was supposed to with that much clearance. I'm assuming that it was just wear from 72K miles of use.
Luckily, I found two NOS yokes on eBay, reasonably price, and included them in the rebuild. Since the tranny was off, I also replaced the clutch. Well, after getting it all back in, the tranny purrs, the vacuum shift works fine, and it hasn't jumped out of gear anymore!
I'm pretty sure you can pull the floor cover and access the transmission cover without taking it out to check your yokes. It may or may not be contributing to the problem. Others can probably chime in on what the specs should be, and you can probably replace the yokes without pulling the whole transmission out.
Please bare in mind that I have no idea what I'm doing half the time. I have a Fulton shade on my Fleetline...
