I will draw back and while holding and aiming the string will jerk forward, and I think it is a matter of not hitting the back wall (too long of a draw length).
That hit the nail on the head as far as what I was thinking.
To me it sounds as though in order to anchor properly [for you], you're allowing the string to creep forward and not hitting the wall and holding it there. This causes the string, as you said, to jump forward.
My question is, who at the shop that you purchased the bow from allowed you to walk out of the store with a bow that wasn't properly fitted?