Are you just using Ruger rings or an adapter. The most common cause is normally the base or rings shifting, so how did you check it? If you just wiggled it that may not be enough, I've seen bases and rings that felt tight but still moved under heavy recoil. Are you loading CP's yourself or buying the Grizzly ammo and if so is the powder or ammo from a different lot # than the previous. Also I would doubt the scope is failing but with a scope that heavy on both ends and only two rings damage can result faster than it would normally from scope tube deflection. I run the same scope on my Encores up to the 458 Win and haven't had problems but that's not to say you might not. They make two types of Weaver adapters for the Hunters, one is a standard length base and the other goes up to the front sight, that may be something to look at if that's not what you already have.