Your scope is good quality, but you never know, it could be broken.

I would first suspect the mounts/rings. Swap them out and start over. Clean/degrease the new mounts/rings and the screws, and use lok-tite (the kind that allows removal later).

I know a guy that had to really tighten his 870 barrel onto the receiver to get accuracy. He used some locking pliers to get it really tight. I'm not recommending that, but do make sure your barrel is tight.