Auto focus hunting problem.

Over the last week I've noticed my 30D with the 17-85mm lens hunts alot to get focus. More often than not it will just jump around trying to focus and I need to switch to manual control.

What would cause this? Sadly I don't have any other lenses any more to see if the same thing happens with them.

Any ideas on a cause and/or fix?

What are you trying to focus on? AF system works on contrast; if you try to focus on things which lack contrast/detail such as a plain sky, wall, shirt etc. it will likely struggle.

Also, what sort of light are you focusing in? The max. aperture of f/5.6 on the long end is the limit of what aperture the 30D's focusing system works at anyway so AF isn't going to be anywhere near as fast as what you'd get with f/2.8 or wider.

I once owned a Rebel XT that did that. It worked fine, and then suddenly it hunted and hunted. It would occasionally lock, which had me tearing my hair out.

Sent to Canon, came back fixed.

I've been using it in high and low light situations, on a variety of objects. For starters the outdoor Christmas concert at our kids school in bright summer sunshine.

It has usually been fine, but over the last week or so it seems to started having issues.