Focus hunting

I've recently picked up a used EF 70-200 L f4 IS. Everything looks very nice and I've taken few photos with it, but for some reason it just keeps hunting for a focus.
My 17-55 on XSi body focuses fast and dead on, while under the same condition and lighting 70-200 keeps hunting.
I've tried photographing a cat and I've tried focusing on eyes and on the nose and it keeps zooming back and forth. When I point to the couch where the cat was sitting with no patterns, just plain red couch, it focuses fine.
Could it be I gotta bad copy or it needs to be sent to Canon for calibration?

Thanks a lot in advance.

Check the focusing distance switch on the lens?

Aleness wrote in post #9739321
I've recently picked up a used EF 70-200 L f4 IS. Everything looks very nice and I've taken few photos with it, but for some reason it just keeps hunting for a focus.
My 17-55 on XSi body focuses fast and dead on, while under the same condition and lighting 70-200 keeps hunting.
I've tried photographing a cat and I've tried focusing on eyes and on the nose and it keeps zooming back and forth. When I point to the couch where the cat was sitting with no patterns, just plain red couch, it focuses fine.
Could it be I gotta bad copy or it needs to be sent to Canon for calibration?

Thanks a lot in advance.

yup, the f/4 focuses lightening quick. If you are in normal light and it's hunting, you have an issue.

Maybe you are too close?

I've tried both distances and it doesn't seem to affect focusing that much. I've tried today outside taking photos of some leaves, it's an overcast, but it's enough light for handholding and one shot, it locks dead on, next one - keeps on hunting.
I'm not sure what I can check to verify it's not my trembling hands (well, not really), but a lens.
The only thing I was thinking is to go to a local store, try on an exactly the same lens and see if there is a difference in focusing.

@k4show, I've tried far and close. Same result.