Focusing problem on canon d60

ok, i know there are a few photographic gurus on here... can any of you help me and explain why, when i half press the button, the camera goes to focus, then past focus and comes back to not really focused? i just don't get it ! i'm using the auto setting option and the kit lens on AF.

Are you using a polarising filter? If so is it linear or circular? The linear filters will effect focus.

My camera a Pentax also from time to time fails. But normally when I look at it closely there is a branch or twig in the picture I had failed see.

With mine focus done in camera. Got a feeling with canon all done in lens. It is faster in lens but more expensive and also dirt on contacts can upset it.

Have you tried removing and refitting lens?