Assuming you're using the same bullets as before.. and not powerbelts.....

Have you tried it switching back to pellets? This should be your first test. If it shoots like it did before, then its an adjustment of the loose propellant charge to make it hold groups, then re-zero. If you go back to the pellets you were shooting before and it throws them all over too, then you most likely have a scope problem or mounting problem. Scopes can and do go bad, rather it be a $59 scope or a $500 scope.

Check the scope mounts and rings. Were the mounts mounted with blue loc-tite and torqued properly? Are the rings tight? If you have open sights, remove the scope and shoot the irons and compare. If it shoots well without the scope, then its a dead ringer what the problem is...... the scope.

I've sent many thousands of rounds through Encores and Pro Hunters using the same charges and bullet/sabot combinations, with the ONLY problem being, replacing the breech plug. This you've already completed.