focusing issues on 50d

I have my second 50d and am still finding that I am having problems with focusing . I shoot a lot of hockey and find that probably 50% of the photos are not completely in focus. I shoot at 1/500 so it shouldn;t be camera shake and f4 so I do have a bit of depth of field. I even mounted the camera on a tripod and shot some simple things around the house like a cereal box and found that it did not seem to focus properly. I really like the qaulity of the high iso photos if it is in focus but because I miss alot of shots due to out of focus it is making me think that maybe I need to try the d300. I also have a 30d and the focus rate seems to be better than the 50d. Any comments or suggestions as to what could be the problem.