I have a lens with the same issue. The zoom is a little stiff now and the focus isn't that great.
I tried the front element fix but that wasn't the issue.
Is it worth it to send it in to get fixed? I purchased a new 18-300MM Nikon lens since it has started acting up. Am I better off fixing, or just trying to sell it for a couple hundred bucks? Small scratch on the glass behind the first element. Otherwise it's in excellent condition.