in re setting up my gears on my road bike my front mech has now got a problem, whereby it sometimes throws the chain right over the big ring. It seems that it is shifting it such that to start with, the very edge of the chain is resting on the very edge of the chainring, and then if it falls 'inward' it goes on ok and if it falls outward then it's come off. I'm sure it isn't the limiter screw, as i've got it so that the mech can only move just far enough for it not to rub when it's on the big ring. Thought it might be a bad chain line or the chain being too loose, but don't think it's either of these as albeit possibly slightly less, it still does it when the rear mech is on the larger sprockets. I've tried it at different tensions so many times that i've knackered a cable so need a new one tomorrow. any ideas of avenues of investigation greatly appreciated.