How far off center were the hits at 25 and 50 yds?
Doesn't it sound like it's just a very old gun, with an equally old barrel, that's the cause of the inaccuracy?
An off center hit at the shorter distances can easily be off of an 8" target at 75 yds.
You're only talking about maybe two to four inches.
And that isn't all that strange at 75 yds, if the barrel isn't in pristine shape.
Maybe just using slightly larger diameter bullets is the cure.
Walt Kelly, alias Pogo, sez:
“Don't take life so serious, son, it ain't nohow permanent.”