Toyota Repair: gearbox - 5th gear, toyota celica, car toyota

Question
I have problem with my 2 identical car TOYOTA CELICA year 1988 , 2000 cc 16 v GTI 110kw .
It's funny but the problem is the same at the bouth 2 car . The 5th gear jump out in 0 point, in full speed, or when the car slow speed in 5th gear .
 It's a problem with this gear box in general ?
 I should expect at any other problem about this gear box ?

Answer
This indicates a major problem with the transmission and it is normally caused by a worn shift sleeve or hub, the transmission needs to be either repaired or replaced, the 2000 model may still be covered under the factory warranty, since this problem is affecting both vehicles I think that it may be caused by a habit of keeping one hand on the shifter while driving, doing so will wear the shifting sleeve prematurely and can cause problems like this, there are no chronic problems witht these transmission  so I'm not sure what is going on here.