Thanks for the replies here. This issue has been keeping me awake at night. I suspected the Kenko teleconverter was the culprit. Something that is further confusing is that a friend loaned me his Tamron teleconverter, which works fine with his same 300 mm lens and D300. It did not work for me with the exact same setup. So....is it the lens or the teleconverter? No problem with the lens without the tele, so it must be the tele.