Many decades ago my father had a Model 740 Remington that shot good for years then started your two group but more severe with the shots being about 6" apart. As you described, you would have two 1" groups but 6" between them. The bedding was the culprit and the problem was quickly resolved. I was only 10 years old but can remember it like it was yesterday. I believe that the bedding is binding someplace.