Picture focus question?

I shots hundreds of Sports Pics and have a question about some of my Baseball shots.
In auto mode, my camera will some times focus on the background,(fence, people especially when lots of color present) which makes the batter/catcher blurred (out of focus)?? I think part of it is the aperture setting but maximium apeture and high shutter speed are required for low light fast action shots. Any suggestion for these out of focus pix??
Nikon D40 with 70-200 mm f2.8 lens

Check the focus sensor location. On the D40 I don't know how to set it but on the D50 in a photo taking mode if you use the 5 way navagation pad on the rear of the camera body (it gets bumped) then the area that it will focus on will change from the center to one of the 4 other areas in the view finder. Simply change it back to the center if this is the problem.
#2 could be that the sensor inside the camera itself is damaged.
Try to lock the focus on a area (usually by depressing the shutter button half way) then move it around to make sure that the camera will lock onto a object.
#3 are you sure its focusing on a different area and not a cause of blur from camera movement. Try to shoot it on a tripod or on a desk or a table.
Other than that I really cant think of anything else that would cause it to do that, other than physical damage.

Thanks

Let me know what works.. I am curious as well.. The D40 I have no personal experiance with. lol