YZFR1 Second Gear problem....

Does anyone have any ideas about the weakness of this particular item, or how
to 'cure' it? It seems to affect the '99 model ('jumps' at high acceleration
in the upper power band), though not the late carb or injection models.

Will the later (stronger) gearbox match the early '99 engine, or is there some
conversion kit, or uprated part that can be fitted to the early box, to beef
it up? Just got a second hand box at vast expense, which is even more
knackered than the original!
Cheers
Martin