First of all, clean the rifle thoroughly. The bore should be shiny, even and smooth for the full length of it. Looking though the bore at a light should help. Check the crown of the rifling to make sure there are no obvious gouges/cracks/uneven wear in that area.

Double check your barrel to stock fit to see if there is now some uneven contact. Double check your scope rings and mounts to see that all is secure.

Clean the crap out of your breech plug making sure it looks as near to new as you can achieve.

Before loading with some NEW powder, make sure the plug and bore are completely oil free and dry. Use the exact same technique each time you load, swabbing first damp/then double dry between each loading. Use the same pressure as best you possibly can each time you seat your projectile and always seat to a witness mark.

That rifle should certainly do better.