If it's jumping out of gear it'll be one of two things, either:
1. Gear selector fork(s) are damaged and not engaging properly, or
2. You're not engaging the gear fully - does your gear pedal need lowering slightly so that you actually move it further with your toe? If it's jumping out of more than a single gear it's unlikely to be the selector, unless you've been really brutal with it. Try adjusting the pedal position downwards and see how you get on.
What are you trying to adjust with the clutch? That'll have no effect on gear selection, just ensure that the correct amount of free play remains in the cable, and the pushrod mechanism (if it uses one).
