Make sure the barrel nut is VERY, VERY, VERY tight, so there is no play between the receiver and barrel. I have seen situations where the nut that holds the barrel on was not tight enough and allowed slight movement of the barrel. On a scope mount that attaches to the receiver this will cause a extreme change in point of impact. It can also be the cause of poor accuracy from cantilever mounts, but should not be as extreme as the op described. If I remember correctly there was a gunsmith making custom pump slug guns and welding the barrel/action together to eliminate the barrel to receiver movement.
Is the scope mounted to the barrel (cantilever mount) or mounted directly to the action?
Good luck, Tony