When you replaced your recoil spring, did you replace it with a Benelli recoil spring? My experience with Benelli spring has been poor. The Wolff 25% increase spring works far better than the original on every application I have used. M1, SBE, Montefeltro. Caution, I do not shoot 7/8 low base shells. Are your shells getting wet, (swelling)? Are you cleaning the recoil spring assembly yearly? Are you lubing the assembly properly?
Patterning problems: are your sights original? If you installed a TRU Glow or similar bead on the gun, it will shoot low because the diameter of the sight is larger. The further you pattern the lower it will be. I tried this with a M1 field, it will not work. No one wants to compensate this by shooting at a different sight point. I am installing a ATN digital sight mount on a Aimpoint saddle mount for my M1 Turkey Gun.
Hope something here will help!