Manual transmission pops out of 5th gear what are possible causes
synchros are shot, or if lucky broken motor mounts
If you are lucky, bad engine mounts. If you aren’t, synchros, internal transmission bearings or bushings are worn out and the trans needs a re-build.
It could be the mounts but I doubt it. I believe the stick mounts directly on top of the transmission. Inside the transmission is a ball and spring that holds the transmission forks in the selected gears. This could be worn out or the spring broke.
If the transmission has been rebuilt or is a reman, it may have been left out, Had that happen once.
Do you (without thinking) drive with your hand resting on the gearshift lever? If so, that can cause premature wear of the shift fork and synchronizer sleeve which then lead to popping out of gear.
Another potential cause is excessive play in the transmission mainshaft. Either one requires a transmission out and disassemble fix.