Its best to start from the basics.

Check the incidences on all surfaces, -+ 1 degree will make trim problems.
Is the plane trimmed OK or did you have to add excessive subtrim to any surface?
Are the ailerons/elevators centered when trimmed for flight or have added mix or trim to correct neutral flying?
Check the wings, ailerons, stab and fuse for any twist or washout created from storage or repairs or temp changes. Make sure all is squared off and measure twice.
Check that no mix was added in the radio, where elevator may activate some rudder deflection.
With big deflections. 40-60% or more expo is ideal.
Do lateral balance, many skip this step, it will matter.
Seal the hinge gaps.

Once the above is satifactory, only then play with the CG, often 1/4 or 1/2 inch shifts will make very noticable changes for the good or worse.
Often just little things can make huge improvements, cover as much as possible.
Load test the ailerons too.

Having the elevator connected on 2 servos is a potential problem.
I would be inclined to split them completely.
Make sure the hinges are not pulling out anywhere.

Can you post a vid of deflections on ground and a quick in flight outcome of the snap?