Ime sure some one with experience of this lens will come along but the obvious things to me are,indoors or outdoors,lighting and camera settings,plus it depends on what your expectations are for this lenses ability,so a little more detail may help.
If it works for you, just use it as it is.It done it on the previous owner, a friend of mine indoors. He said he used it and it was fine, then it set awhile before using it again, and it started acting up. He takes super care of his stuff so I know it's not from abuse. For my needs, which is portraits, I only use single, center focus, and it works fine for that. I gave him $300 for the lens knowing the continuos focusing problem, because again, I don't use it for sports. Just curious as to what is causing it. I've not tried it on my camera in continuous focus, so it could have been his body, D7000. I also will be using it on a D7000 along with a D200. I tested the single, center focus and it is flawless on my body.. Might should check the continuous on my body to rule out that he have been a problem with his camera. Figure if I did end up having it fixed, and shelled out a few $100 for repairs, I'd still be ok with having say, $600 in it if it does have a problem and cost no more than $300 to fix it. But as stated, I'll only use it in single, center focus.
Sounds like it would be a camera problem to me.
If it works for you, just use it as it is.
Test it on your camera no sense in speculating if it works for you. I would clean the contact before use.
These are just questions to be aware of for a good understanding, I'm not looking for answers.
What focus mode are you in? ( like single or 3d or auto area or dynamic & how many points?)
What focus servo mode are you in? AF-S, AF-C or AF-A? AF-C on most bodies has 'release' or 'focus' priority associated with it a menu item. Often complaints of focus problems in af-c are due to folks not understanding that the shutter will not wait for focus in af-c if release priority set. This is further muddied by a menu setting for focus tracking lock (a3 on d7000) which can introduce a delay to continuous focus.
AF-A is switches from af-s to af-c but few people have a like for it. It is really not clear what mode you are in when you use af-a.
I know I babbled a lot here, but take the time to understand the details of continuous mode and you will be in a better spot to evaluate your lens. If you are looking for a quick-start, then try single point mode, af-c and set for FOCUS Priority. That would be menu A1 on your d7000. Set menu A3 to off. These are not necessarily the best settings, but you should be able to get a solid focus with these settings. There are two basic things that happen with focus. First is how does the camera acquire a focus point (single, dynamic, etc). Single point will keep it simple. Second part of the process is what it does after identifying a focus point, which would be the servo mode and release mode. Focus priority will make the shutter wait for focus so you can observe lens behavior. The shutter will not fire until you have focus.
Hopefully your lens is fine. Good sign that it behaves normally for single focus.
Is there a best way to clean the contacts? I've never had to do it, but have read and heard of people doing it with a pencil eraser.