True, you'll find variations. However, the fact we're talking 8-10" groups at 100 yards is well out of the norm in and of itself. Something's seriously wrong, and if it's not fixed, no amount of tinkering with different loads is going to change things.
Agreed. I've had rifles dislike a particular load and shoot 3 to 4 inch groups at 100 yards because of it. But 8 to 10 inch groups? Man that's got to either be a shooter or equipment problem there. I'll join the chorus. Recheck everything for tightness. Take someone else to the range with you that you know to be a good shooter.