1. Are you using an M2 type bipod? If so, you may have your culprit. I've found that when using the M2 I can have wild groups, especially if I am putting pressure onto the bipod. It's an MG bipod, not an accuracy bipod. So, if you're using that type, stop.
2. Sight picture. Are you sure you're choosing the right sight picture every time? For my part, centering the target horizontally is far easier than centering vertically. That could just be my eyes, but at 100 yards I have to really concentrate on the sight picture I want, and then concentrate to be able to release a round when I see it. Vertical strings have not proven uncommon in my experience. But then, I'm no NM shooter, so YMMV.
In sum, it's my bet that the culprit is you (not in a bad way) just like almost every time something is messed up while I'm shooting, the culprit is me.