Look here about 7 or 8 posts down `four sped shifter`. Mine used to do that too and the first time it happened I nearly panicked I was on an exit ramp and it was a steep one climbing up. I turned the engine off, and was ready to leave it over on the side until I saw a police car giving me the eye.It wasnt easy getting that car going in 4th gear up that incline

Eventually I got so god at putting it back together, I woulkd just coast over somewhere there was enough light if it was dark, put it together and take off in about a minute or less.
After I had the transmission replaced later it never came out again,I was thinking it was the nylon nut that screws into the transmission, but I guess it wasnt. It could be the saddle or shift rail bushing was missing, or maybe grease on the nylon nut (or in the transmission housing where it screws in) or else the transmission itself in my case. If you get the saddle then you might want to clean that nylon nut and the threads inside the top of the transmission a little too. And let us al know how it turns out.