Originally posted by crlester is it because the lens is faulty or is because its an old make and cannot transfer data to the camera quick enough.
Try the same type of shot at 50mm or so and see if it makes a difference. It is likely that on the long end the camera is not locking focus as fast as it would with the shorter kit lens. Some lenses seem to be faster to focus than others. I've not used that lens but I do have a FA 28-105mm that is remarkably slow to focus compared to my DA series lenses.
And it might not be anything wrong with the lens at all. Focusing at 320mm is going to be harder than focusing at 50mm because of camera shake if nothing else. See if the lens focuses quick enough at short focal lengths first. If it does then the problem is just the long length. If the problem persists even at 50mm then I would say it is the lens.