Hi Silversurfer,
Good to hear from you and have your thoughts.
You sound like Sherlock Holmes, "it's elementary dear Watson!"
Good advice, if the plane has a problem without differential, try
using some.
Does not cost a penny and can do no harm.
If it helps what care the aerodynamics that did it?
That is of course, if all you wish is a correct flying airplane.
Be good,

Hal deBolt