If I am doing a rocket gearbox, which is not often really, I make the shims needed. The gears often have too much end play. Ten thou end play is ten thou less dog engagement. It all adds up.
Maybe I should attempt to have the shims made to measure. @Neville Lush, what tolerances did you work to? How much end play did you allow? "Pretty please, with sugar on top."
Mate i know how much you baby your bike, so im floored that your gearbox has let go with so low klms, must be treat em mean keep em keen at work here, i dont exactly flog mine but it's had a few slappings and no similar issues knock on wood.