it might also be from the aileron differentials.
more noticeable on high wing mount planes.
there's a couple of theads that goes into details..but i can't remember where.

in other words... the ailerons are set too low.
they'll act like air breaks, cuasing yaw.

havn't experience myself, but a bubby of mine was
having semilar problems on an under power bird.
he fixed it by raising the ailerons.