Hmmmmm. Have you balanced the model laterally? That is, tied with a string around the prop shaft and between hinges of the rudder it hangs level along the centerline, without one wing dropping consistantly before the opposite side. You should also carefully measure from wing-tip to the top of the vertical stab from both sides and make sure that measurement is equal.

You can correct some barrel-roll tendancy with differential aileron mix (more up than down throw) but the tendency for the tail to advance on one side is new to me. You've built Lomcevak into it somehow! Neat trick!

Does the tail swing to the inside or the outside of the roll? If the tail droops towards the ground it is reacting like it was extremely tail heavy. That, or the rudder & vert stab alignment is unequal and it is tucking one side once it has rolled enough that the rudder is providing lift (or anti-lift). Check the incedences of ALL the fixed surfaces and not just the control surfaces.