Most focus mechanisms have a slip fit in the focus rings. This allows for extra force applies at the end of the focus scale. This can loosen up over time and will cause the focus ring to slip at the wrong places. This really isn’t a user serviceable problem and should be repaired by a qualified technician. You may want to check locally for a camera repair shop in your area,or, if you need assistance finding a shop, please let me know and I'll locate one for you.