If it's a recoil operated gun, you really have to hold it stable and solid. Otherwise, it's like limp wristing a pistol. I'll be straight up with you on that.
I'm not saying your a wussy, but, rapid fire may be interfering with how it works. Next time you get to shoot it, take a handful of shells and shoot it slow and deliberate and see if it improves.
Honestly, if it were me, I'd take 3 different boxes of shells with me next time. I'd load up a mag of high brass, shoot them out (or try to). Then a mag full of low brass or intermediate loads, and afterwards load a mag full of handicap loads through it.
If you can get all 3 working, I'd randomly load the mag up with all 3 different kinds at the same time and then see what it does.
As I know you are aware, not every brand of shell is the same. I have some 1350 fps winchester and 1350 remington shells. The remington will always eject another 12-16" further away that the winchesters. Though that shouldn't surprise anyone. Winchester has never really performed as well as remington in the real world once you get off of paper.