Please do not take this as insulting, but have you checked for leading in the barrel? are you shooting Lead bullets? or jacketed?
What does the crown of the muzzle look like? sometimes a small ding there can really upset accuracy.
have you tried shooting off a couple of sand bags, bracing your hands on the bags? this tends to remove any shooter induced errors.
Have you made sure the rear sight is still tight. My dad had a K 22 that he said just went south real fast, turned out that screw holding the rear sight had gotten loose and with every shot, there was a new setting. just some ideas?