I agree with Chris B....
Check the LATERAL balance... I'd wager the Left wing is heavier...
and that will cause all kinds of headaches...
another real killer is having the control surface gaps uneven. if the airplane does not track well through loops,
balance laterally, and seal the hinge gaps (if not already sealed.)
Hope that helps!

(BTW, the term "Coupling" is when you command one flight control, and the result includes a second <uncommanded> control input)
when in knife edge flight, (with lots of rudder) it's common for the airplane to need elevator input to keep it flying straight....
the cause of that elevator input requirement is Rudder\Elevator coupling.