Hi. Sorry for delay in replying, I've been at Sail Caledonia!.
Is it i) your sail leech rapidly flapping or ii) your Jib Boom slower oscillating up and down.
If, as it sounds like, it is the Jib Leech, then you could try tightening the adjustable leech line, which I have on mine. I think this should stop it, but be careful with the sail shape as you don't want it hooked up at the leech.
If it is ii) then I have more info available.
Jim