yes, they were prone to it, you used to have to weld the new nut in place.
but was it not from 40k to 30k that was changed on some of them instead of the amount of gears.
jpt, check the syncromesh cluster, iirc there is 3-4 small fine plates with ridges in them that the syncro shifts on, there is two spring clips made out of wire as well, these can go weak, they basically hold her in gear til you change, altho cycles is right about the plastic on the forks as well but you say they are good.
if the teeth are worn on the main gear, even slightly it will cause her to slip out as well.
was she grinding into these gear's at any stage?