Karl, oddly, I've had similar problems with my factory .25-06 Encore barrel. I've tried many powders and bullets. Every time I think I have it figured out, I take it back to the range a few days later and the shotgun pattern emerges. I've only used bullets from 100-120 grains from Horn., Sierra, Nosler, Speer, Barnes. All get weird.
The only powder that does half way decent is IMR 4831.
I think the gun has a jinx on it.