[Has anyone been sandboxed before and what did you do about it? First, let me say that there is no Sandbox. What you are likely seeing is simply competition outranking your web pages.]
The reason you sometimes see new web pages quickly climb the ranks, only to drop after a short while, is usually due to the temporary boost of QDF. This is a temporary boost, not a penalty.
My advice would be to stop worrying about why you are sandboxed because you are not. You are probably ranked right where you deserve to be ranked based on the strength of your competitors. Instead, focus on earning rankings through solid promotion efforts.