My brother owns an '87 Nissan Pulsar NX and when he is driving the manual transmission pops itself out of 5th gear. I've heard of this happening to other people, but I don't know what it means or what needs to be done to correct it?
This was a common problem with some 1987 and 1988 Nissan Sentra and Pulsar NX vehicles with manual transaxles. The condition was due to premature wear of the 5th gear shift fork which causes insufficient gear engagement.
This will require disassembly of the transaxle and replacement of a group of internal parts including the shift fork, the 5th input gear, and several other items in order to do the job correctly.
Take your car to a Nissan dealership or a qualified transmission shop. They probably know all about the repair, but show them this Nissan Technical Service bulletin number (TS89-064A) just to be on the safe side. It's a significant job and will cost a few dollars. They may suggest a swap with a rebuilt unit.