Larry:
Typically a problem of popping out of gear is associated with bad clutch teeth on the gear and/or a synchronizer assembly that is getting sloppy. On most used transmissions, second and third are normally "throwaways" gears as they take the bulk of the punishment. Fourth is usually suspect, as well.
Since these units (gears and synchro assemblies) are relatively inexpensive, it makes sense to replace them when you tear the transmission down. Replacing the input/mainshaft bearings as well as the brass blocker rings is also inexpensive and should be done. You might want to have your mechanic contact David Kee
www.4speedtoploaders.com for the parts, as Ford no longer carries them. Good luck.