there is a Detent Ring on the syncro that the slider fits over aswell that could be worn or the slider could be jacked up gotta pull it aprt to see.. soo do the easy stuff first, also could be a shift fork issue. Id pull the oil plug and check fluid, see what it looks like, try to flush it and start with new fluid see what happens. When my Trans was slipping out of gear it was, the shift forks being seriously worn by lack of lube from a busted seal on the Expressway, but also my 5th gear Bearing blew apart. it still shifted good, but i replaced everything moving in that trans.

 

Replaced fluid with new Mitsubishi oil car is like new