My first guess would be that you have an issue with the stock. The first thing I would do (other than replacing it) is loosen both action screws, stand rifle up on recoil pad, give it a bounce on the floor (use a phone book or something similar as a pad), and retorque the action screws. Use a torque wrench and don't guess. When torqued then take a piece of paper, fold it once, and see if it will pass from the front of the stock back to the action, under the barrel. If not you have interference from the stock and it needs to be dealt with.
Rather than focus on ammo, focus on a stock that on it's best day still has more flex in it than it should. It was really designed for firing one or two shots on a hunting trip, not a day at the range target shooting. The warmer it gets the worse it will be. Hint: Notice how it got worse the more you shot?