Hence my caveat about mount type. The 3-screw std would be pretty wiggly with a loose base screw and/or windage srcew. Still, since you're still having trouble, something else is clearly wrong. Can't imagine a mildly pitted bore throwing shots that wildly. Replacing the scope is a quick way to eliminate one possibility.
The collimater should show if the reticle jumps around, I would think. Also, come to think of it, a cracked stock or REALLY bad bedding might cause your issue, but the huge jumps of POI are striking, no pun intended.
