There can be any number of causes: too much hinge gap, slop in the linkage, slop in the servo, to small (limber) push rods, worn servos and on and on. For big birds, you must pay attention to all the above. Have very stiff linkage (no z bends or cheap hardware), sealed hinge gap and also helpful are static and dynamic balance of the surfaces although this may be difficult to do on that plane. A debateable item is to NOT have any rounding of trailing edges, leave them square and blunt.

The modification probably isn't the only cause of the problem... but the added area gave the control surfaces more leverage to fight against the servo.

Check that the control surfaces theselves aren't flexible.

Make sure the control horns (or torque rods) aren't able to be moved when the control surfaces are held in place. CA soaking the balsa at the control horns, or injecting per-mixed epoxy around the torque rod inside the control surface may be needed.

Assuming you went with the stock single servo on centerline and torque rods... you can widen the servo opening and put two servos, side by side, giving more power to each control surface. Or.. upgrade to a higher power, ball bearing metal gear servo, to help take on the load.

The condition you described is called "Flutter" and it can get bad enough to rip the wings right off the plane in extreme cases.