I've had this D70s for the whole summer and it's been great - no shooting trouble at all. I'm using a 60mm lens. All of a sudden now, when I try to autofocus, it will sometimes just focus completely wrong and pass by where it should correctly focus. I'll see it come into focus but it'll just completely pass by and go back out of it. If I try to focus on other things it will work, but half the time it just either focuses all out of whack or doesn't even try at all. Any ideas why this would happen?