[digitalfailure]

What aperture are you on here?

It does sound more like a defective lens to me.

When you re focus using the manual control, your using your eyes and noggin to determine the correct focus. The camera isn't doing anything.

download a focus testing chart from somewhere and set your camera up on a tripod in a room with average lighting. manual focus on the chart at 18mm and take the shot using a remote release or self timer, adjust the zoom out the the end of the lens and focus lock, before adjusting back down to 18mm and selecting AF.
Once the camera has gained focus lock on the same point at the same focal length, take the shot again.

Compare the 2 shots and see which is sharper.