'93 Sentra pops out of 5th gear.

[2.3Turbo_T]

I'm brand new here and looking to purchase a '93 Sentra w/1.6 transverse 4cyl/5spd. The car has 198k but seems to run well and everything works. It just recently had a new clutch installed although it lets out fairly close to the floor. The problem is when I shift into 5th gear on the freeway, it immediately pops out of gear. I can keep it in gear by physically holding the shifter in place but it feels like it's just barely engaged. I've been told this is a common problem to these cars and it may be an easy fix. Could this be just a linkage issue or is it an internal transmission problem?

It's gonna be a deal breaker on the purchase of this car so any help is greatly appreciated!