Just a couple of questions and possibilities.
- Does it happen with a particular lens, or any lens?
- What are the light conditions when this happens?
- What kind of batteries are you using?
- What AF mode are you using?
The DS/DL series is known to have some issues locking focus in lower light conditions with darker lenses. If you have faster lenses, see if you get the same behavior with them.
NiMH batteries that are somewhat discharged can generate some less than ideal AF behavior. Try Lithium AAs or CRV3s and see if you get the same behavior.
Try using the center AF sensor only.
If none of this helps, or your problems are more random in nature, you may have a camera problem, and it should be checked out.