Number your arrows so you can tell which one does what. The flyer may be an arrow with a questionable spine and by numbering them you'll be able to tell if the flyer is always the same arrow. Do you spintest with the broadheads on? If so, did the broadhead point stay in the same place? I mark an index card and hold it at the broadhead tip to see if the tip "wobbles". If it does wobble you may need to square off the insert or you can try a different broadhead on that arrow. It may be a broadhead that is causing the flyer.