It's most likely an incidence problem, wing or tail, although the wing would probably be easier to adjust.

Had a Waco like that, flew well all it's life until I sold it, didn't see a reason to fix it, plus it was harder to do on a bipe.

Check your wing seat, make sure the wing is sitting in it's intended angle.
The CofG will affect your elevators efficiency not it's level flight position, unless you have wide differences in elevator trim for different throttle settings.
Is this the case?