hey i dont know if you've ever heard of your direct drive gear, but all standard trans have them most are usually 4th gear but some are 3rd. Possibly urs. It is possible for ur direct drive gear to pop out when accelarating and hit the smallest bump that it will pop out like you explained. If it is that direct drive popping out like that than its not a problem at all. Basically can say sh*t happens. Now if it keeps happening, you may want to get it checked, possibly ur blocker rings or clutching teeth or even ur synchronizer assembly. All 3 of which(along with other parts of course) make it possible to shift and hold in gear.
I reccomend doing research on figuring out what gear is your direct drive. If someone tells you there isnt a direct drive, dont believe them. They are simply dumb! Hope this helps--------------Tony