Hi, son just got given a "new" work car, a 4cyl TR manual sedan ~230,000Km. It jumps out of 5th gear while your driving. I had the same problem with a V6 manual Wagon at work years ago, it eventually spat bits out the bottom. The dealer said at the time that a nut that holds 5th gear on comes loose. Could this be the same problem, this is a late 94 sedan the wagon was probably a 95?
PeterN

its 5th gear syncro / box out to repair / or fit a s/h better box .

not really safe to drive like that / do as soon as possible

yep problem never really solved by mitsubishi motors

There are three plugs on the side of the gearbox, under them are springs and balls, these are meant to keep the car in whatever gear you have selected. Consult the manual, it may be that the spring is stuck or broken, or the ball has disintegrated. Also check that the shift cables are adjusted correctly.
Otherwise, it is a pull the gearbox out and dismantle or replace job.