Gearbox trouble

OK - just when I thought things were good. Only minor troubles, then gearbox starts acting funny. It keeps jumping out of third gear. Not good. I (naively) suspects the gear selector forks - either worn or bent. Will try to fit another top with good forks. If that does not do it - I will have to dismantle the fbox for a rebuild. Till then I just have to do without 3. gear :-(