Whereas my Super Kaos DOES 'wriggle'. Something to do with airflow breakaway from the fin, and does affect lots of aircraft, model and full size. There are cures, most are simple, thickening the trailing edge of the rudder will do it, a simple litho angle attached both sides of the trailing edge then trimmed to suit is a common one. That being said, my wriggle is only apparent in windy (turbulent) conditions, and is not visible in calm weather.
Evan, WB #12.