It may not be the gun. It could be you.
You are just settling in to shoot with the first shot.
It sounds like you have eliminated several of the most common problems. So try this. Shoot, get up and walk a little bit, shoot and repeat. See what that does to your patterns.
I usually throw the first shot off the bulls-eye by an inch or two on my first shot and then can settle down and shoot very well.
If this does not work, I really do not know what else the problem could be, as the guys have pointed out above, it seems you have everything covered.
Inside Every Bright Idea Is The 50% Probability Of A Disaster Waiting To Happen.