Just went through same problem with 12ga/scope sighting in problem: found the recoil had moved the scope (tight but not tight enough) and the hammer was hitting the scope but it would only do it like every other shot or so. am going to shim the scope and tighten better with RED loc-tite.
Edit, Sorry went with blue Loc-Tite, and a double back tape inside the rings themselves. Should'nt have any more problems. thanks Tim.