Nine times out of ten, I have found chain jump IS caused by a STIFF(TIGHT) link in the chain,Take chain in both hands and bend it sideways a few times,Even new chains can have a few stiff links ALSO alignment of the rear cogs and the front chain rings,can cause this, Use spacer washers between the block and hub to bring the block out a bit, this can help, Cheers John Crump OldandstiffBrit, Parker.Co USA