I bought a D7000 several weeks back. I had been using a D200. It's a considerable upgrade from my D200, so far I'm really like it.
I had a Nikon 24-85 f/2.8-4 lens that had some common soft focus issues. I finally sold it and bought a Tamron 28-75 f/2.8 lens a few days ago. I am having problems with the lens staying in focus with the D7000. I'll have the picture in focus ready to snap a shot, then it will either start refocusing over and over or stayed focused and then jump out of focus until I hit the shutter button. None of my other lenses do this on the D7000. I put the Tamron on my D200 and didn't have this same issue.
Is this a bad lens, bad lens/camera combination, or something else?
#3. "RE: Bad lens, bad combination, or something else?" In response to Reply # 2
I truly do not know anything about the specific lens in question, nor the D200, but I also bought a Tamron like half a year ago, a 70-300 DI VC. Sadly the lens had severe problems focusing on my D7000. It "hunted", went back and forth all the time as long as I held the shutter release halfways, even if I had it on tripod with a static subject. So I went back to the shop and had it exchanged for another one. What a world of difference! AF now works like it's supposed to do, and the VC (Vibration Control) is like locking a missile!
So, maybe your lens is suffering from the same kind of problem I had with mine. I suggest you just go back to the store if possible, and try have them to give you another one as it is a recent buy. And btw, ask to try the new lens while still in the shop.