It doesn't happen on the GA16DE maunal trannys as fat as I know, just on Sr20DE trannies. It pops out of 5th gear easily when you're driving. Nissan blames it on a bent shift fork, caused by people resting their hand on the shifter while driving in fifth gear. I don't know of any specific mileage when it starts. I can't remember what it costs to have fixed either, but it's not cheap at all.
Applies only to 1991-1993 five speed transmissions attached to SR20DE. Nissan claimed bent shift forks from drivers resting their arm on shifter. That's a bunch of shiite. For 1994, Nissan redesigned certain fifth-gear parts in recognition of the problem but wouldn't perform a recall or acknowlege any design fault. By now, most early 5-speeds have been rebuilt or replaced due to this "feature".
I got a 91 se-r that came into the shop yesterday with 5th gear popout. On this one the shaft bearings were wore and allowed slop in the meshing of the 2 5th gears causing it to pop out of gear. It needed 1 of the 5th gears and all 6 bearings for wear, an axle seal, shift shaft seal, and leaking backup seal. The estimate was like $1800. The customer ended up getting a new trans. I'll be putting it in tomorrow. Oh, guess what? I now have me a spare trans. whhoooo hhooooo!
I had 5th gear pop out on my 93 E (B13) and it sucks. At first it would pop out from time to time them it got realy bad where it would not stay in 5th. So i drove around for a while holding it in 5th when i was on the free way. Then i got tired of it so i took it to AAMCO and it costed me about 1989.00 It was still cheaper than a new trany from Nissan. I highly recomend that if you have this problum then get it fixed right away and dont wate it wont go away.
Its still cheaper than buying a new car. I only put in 5th gear around 60/65 MPH. Any thing less it be comes slugish.
I think it all depends on how you treat your trany.
A new tranny from Nissan w/ the upgraded internals cost me around $1800 from Nissan back in 1997.
I do not use 5th gear until i hit 55mph in normal driving which is rare on city streets, basically 5th is just for the freeway.