I have a Stingray that I purchased new at Guitar Center in April of 2004, and have noticed that pickup constantly pulls itself towards the strings. I can adjust it, and it will stay for a couple of weeks, and then I start to notice a chorus like effect when playing highup on the neck. The strings will almost be touching the pickup, and actually click off of it using a fairly light touch. So I lower the pickup and it will stay for a couple weeks, but then I am right back where I started. Is this normal?