The wheel on my old MX-518 is acting up sometimes. Suddenly the wheel stops working, at least scrolling down. Scrolling up works at least to a part. However, if I keep scrolling up for a while it suddenly starts working again for a few days until it happens again. Now, I don't think there is some kind of max mileage on the wheel... but it certainly seems like it. I guess it's just coming to an end, i just find it a bit strange.