May sound stupid but. Any cracks in stock? Any loose screws. Is scope tight? Is there a dollar bill space between barrel and stock? If not stock may be too tight on barrel and as barrel heats up it changes point of aim. Remove the bolt and place on sandbags and sight in target thru barrel. Now look thru scope, are they the same? Finally are your crosshairs broken. I had that happen on a shotgun. It was dead on. Then every shot was different. Under close inspection crosshairs broke in two places.