Focus hunting.

My 30D and EF-S 17-85mm doesn't like auto-focusing at the moment. It sits there hunting back and forth like crazy, and then fires off a shot when it thinks there's a lock, but it's usually way off.

Light levels doesn't matter to it.

What could be causing this? is it likely to be a lens or camera issue?

this one sounds like both of them got some issues
they don't like each other -_-;;;

You might try cleaning the lens contacts with alcohol. It's worth a try.

Have you tried resetting the camera back to factory defaults? Which focusing mode are you in? Which focus point(s) are you using? What sort of things are you trying to focus on? Have you any other lenses or bodies to try?