I have a nikon d60 and a tamron af18- 200mm f3.5-6.3 xr diII lens. From time to time, the autofocus will not work and occasionally the camera will read "lens not mounted". With the autofocus problem, I have to turn the camera off and back on again but with the lens problem I have to unmount and remount the lens...any ideas? Bad lens or bad camera? Thanks for the help