Problem w/ 16-85 autofocus- input pls?

My 16-85, recently purchased used (returnable). 
Will not lock on focus at widest FL. At 16mm the distance scale goes to infinity and the focus indicator shows it's front focused. Now, if I zoom to just past 16mm, just nudging just off the widest FL, the AF-S works fine and achieves focus. I've tested it in low light, outdoors, different metering modes...same result. I'm using a D7K, my other lenses are fine (e.g., my Tamron 70-300 VC has no issue at 70mm, my Tamron 17-50 has no focusing issue at 17mm). Don't know if it's relevant, but I've cleaned the contacts on the lens.
Anything I'm missing or should try before I return the lens (user error?).