Ed and Jerry are spot on with this tranny problem. Linkage,internal bearing wear, or a worn main shaft will cause the tranny to pop out of gear. Had an early Muncie 3 speed in my 1958 Chevrolet that would pop out of 3rd. Had a so called tranny shop rebuild it and it still did the same. Rebuilt it myself with all new bearings and main shaft, adjusted the linkage to FSM specifications and no more problems.
Tim
