Decreasing the wing load also helps. Add more wing area or reduce the mass of your plane

You probably have too much elevator deflection or your elevators are too big. Try making them smaller or decreasing the deflection.

Use mouse as joystick for smoother controls

The wings might be the problem....
It must be too long that it flaps up and down
To compensate this compose the wings with structural ones and the last bit should be primary wings for the roll
If that's not the problem then I don't really know....