Sounds like a very similar road to one I just traveled. Assuming steering is all good and you're not trying to run at more then 3/4 trim:
1. Start with the prop - make sure it's good. Have someone else run it on their rig or borrow another that is known good.
2. Verify that your jackplate is set at exactly the same on both sides. My manual plate would bind more on one side when adjusting, resulting in the motor being a bit cockeyed and consequently not lined up in the center of pad. G's don't like that.