The Autofocus function stops working intermittently on my D300s. It works ok for a few shots then shuts down. If I leave the camera off for awhile it works ok again for maybe 20 shots, then stops again. I use the AF-ON button for autofocus. I had taken about 300 shots immediately before the problem started & wonder if overheating might be an issue. As I am shooting wildlife I find manual focus a bit too challenging for moving targets. I am using a 300mm F/4D with 1.4 TC, but doubt that would be an issue. Any advice would be appreciated....Scotty.