Got a Nexus for Chirstmas, all my others are X-cells. To make a long story short, first tank of fuel fine, second one got hit with that wobble so bad I almost lost it. So I thought, if everything is blanced what would cause it , simple I had the same problem years back with concepts, The fix, the top bearing block being molded plastic is sloppy. Take a soda can cut a shim to fit around bearing with 1/32" gap between ends remount and presto no more wobble. I've seen this in every plastic heli sooner or later that do not use metal bearing blocks,