Gears jumping out is usually the selector forks gone Killed in Action. You'll have to drop the box to have a look, I'm afraid. If a lot of little bits are worn, you may find it cheaper to get a whole replacement "Exchange" gearbox.
It has nothing at all to do with the clutch (that would only affect gear-changes) but when the box is off, if the clutch looks anything other than pristine, fit a new one. It'll save you doing the job again soon (depending how worn your clutch is).
Ralf S.