New to D300
Remember, unless the airplane has a face painted on the front, the D300 is not going to focus track. Also, if the face doesn't have a big smile the camera won't take a picture..........just kiddin'.
I hope you enjoy your D300, it's a fantastic camera for action.
This is how I use the camera:
I use continuous a lot, the camera is very responsive in continuous mode. Sometimes I use 9 points and sometimes I use 21 points, but I can't see where it helps very much. My keepers don't seem to change. I never use 51 points or 3D. That is giving the camera too much control. I like to decide what the camera is going to focus on, besides I doubt the D300 has enough horsepower to be effective using 51 points. For 51 points or 3D, you probably should get a D3 or D700.
I don't like AF Lock at all. It acts like a time delay and many times the camera will focus on something else and then there is a delay before you can get the camera to focus on your subject again. With AF Lock off, the camera will instantly focus on anything you point it at.
It sounds like multiple focus points and AF Lock are causing your problems. I would try Center Focus Point, continuous focus, with AF Lock off.