There are a number of things that could be causing this problem. It would help in diagnosing the issue if you could be more specific about 'jumping out of gear'. Is it jumping from 2nd into neutral? Is it jumping to a 'false' neutral? Is it jumping into 3rd? Does it do it when the engine is under load, light throttle, overrun etc. or at any particular road speed/rpm?

Worn/rounded gear dogs and/or a bent selector fork are the main culprits with this sort of thing, but as I said, without all the symptoms it will be hard to pinpoint the fault.

__________________