I hadn't thought of the breech plug, but that could be a possibility. My first guess is still the scope/mounts/rings. It seems to me that if your accuracy was good before, then the scope was changed, and now your accuracy is bad, then it's probably the thing that was changed. Maybe try pulling the scope and shooting open sights at 50 yards and see what kind of group you can get.
I would also maybe recommend dropping your powder charge, though if that's the load that been working I suspect it's suddenly going to change.
Those would be the two cheapest tests, so that's where I would start. If you're still having problems then I would look into hinge pin locking springs, etc.