it does sound like a broken gearbox, you could try new oil in it, you could also check the gear linkage is ok. but really it sounds like there is something wrong with the selectors,
i cant remember off the top of my head what causes it, but there is a problem that causes the gearstick to jump back to neutral especially in second and i think it is to do with the alignment of the selectors in the gearbox. but it still means taking the box off and opening it up.
