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Took the recently acquired NB Heritage out for its first car club run today.

All was fine for awhile, then suddenly couldn't get past 4th.

Pulled over and tried when stationary and also with the engine off but still no joy.

Rejoined the group and drove in 4th for awhile and then tried 6th. It went in and then fine going back to 5th.

After that no more problems and drove the rest of the day will all gears working normally, though did notice that sometimes reverse was a little hard to select.

Any ideas about what went wrong?

Any idea when the gearbox last had an oil change?

Good question. I'll ask the previous owner.

Sounds like following, except that when this issue occurs it doesn't right itself temporarily or otherwise: 6 Speed will not shift into 5th or reverse: https://www.miata.net/garage/tsb/s05_112_02.html - However, worth checking that the aforementioned hex bolt is tensioned correctly in case it was loosened in the past, then not tensioned correctly, & is slowly working itself loose.
Other thing to check is the throw of the clutch slave cylinder push rod in case the clutch slave cylinder is on the way out, as they typically last about 10 years.

Here is a video on what the hex bolt does, this guy removed his by accident and had to pull the transmission out again....

https://youtu.be/4Isbc92MJn0

Long shot but it could be that a chunk of the nylon shifter bush has broken off and jammed up the selectors. It might have worked its way free when you were driving so you could reselect gears.

Pull out the gear stick and have a look. Make sure its all together and that something isn't down in the shifter area.

Could be you pulled the gear lever too hard to the right in neutral those few times.
Try it with the engine off & you'll see what I mean.

I was watching a youtube video on this yesterday http://youtu.be/BtmnEJz5fWA hope that works otherwise try "The car passion channel