Grinder67, I may have your answer. Actually not "my" answer, but that of writer Randy Wakeman in one of his muzzleloader forums.
The exact same thing happened to my Knight Wolverine that once shot 3" groups at 200yds (no I'm not kidding).
Mr. Wakeman suggested replacing the breech plug with a brand new one (same model of b. plug of course) and... cha-ching! My groups once again returned to their great original accuracy.
Turns out that breech plugs slowly erode away; progressively changing your barrel pressure ever so slightly.
Hope this works for you as it did for me. jimsmokepole