My Nikon 18-200 lens on a D300 camera will not focus when it is zoomed between 135mm and 200mm. Is there any tips on how to solve this besides handing it in to a photo store?
There really isn't all that much you can do with lenses yourself without making things worse. It sounds like something mechanically is wrong, and the only way to fix that is going to be to open up the lens.
I would take it to a Nikon authorized repair shop. If you are in the US, this form will take you to the list of authorized repair shops.
Just a note. I found this thread while researching the same issue, though the range that wouldn't focus for me was more like from 80ish to 200. After reading the comment by GRM, I investigated my lens and found the front element to actually be loose, having backed out on its threads. As I never take the UV filter off, I never would have found this with out GRM's comment. I tightened the element snugly, trying to resist my urge to torque the crap out of it, restored my UV filter to its rightful place, and bam. Autofocus issue appears to be fixed. Thanks, GRM.
I have experienced the EXACT same issue. I sent it to repair and they said that the top had to be screwed back on. I have also read that this could be caused by dirty contacts so you can also try to clean them.
I have seen some articles about this on different forum so I guess it's a rather common problem and you shouldn't pay anything to get it fixed.