Back in my DX days, I was a heavy 18-200 user (first version). As others have pointed out, f/5.6 is not really very wide open, and mine also did have problems locking AF beyond 150mm, with some hunting.
But it was a consistent "non-problem" from the start: it did that as soon as I was not in bright daylight, and it is not a problem that "developped" over time. Maybe you're starting to shoot in the more interesting lights of early morning/late afternoon? That could be an explanation. Usually, as you note, closer objects are better illuminated and more contrasted, making the job of the AF really much easier than on more distant ones - and zooming back is the only solution, in order to regain a more friendly f-stop that will allow more light to reach the AF sensors.
A simple test/remedy, also, is to try with the center sensor: on most cameras, it will be a cross-type one, more sensitive than the linear ones in the periphery: on the D300 I owned, the three center columns of AF points were cross-type sensors, if I am not mistaken, and thus less prone to hunting in not-so-good lighting conditions.
If it can soothe your "pain", my 28-300 (yes, I like those "do-it-all lenses"!) does also do that in the same conditions on my D700. Using a faster lens such as the 80-200/2.8 or the 85/1.8 is also a remedy: since the lenses are wide open when composing the image and focusing, more light reaches the sensors and AF is really quicker! Calculating the figures, at 200 and f/5.6, the AF sensors get 4 times less light than at 200 and f/2.8, since the amount of light entering the lens doubles with every full f-stop: f/2.8 is twice as luminous as f/4, which in turn is twice as luminous as f/5.6.
Hope this helps!
Olivier Rychner __________________________________________ Jetez un oeil à ma galerie if you feel like it! And it's a bit void as of now, but I also have a Nikonians blog
Auta i lomë! And my Nikon's only awaiting daylight...