Buffering

Hi, I have just installed and playing around with the software. So far so good, just a small issue. The media player buffers the video I am watching quite often during playback. This happens on a few files not all. I have the latest software updates for apple tv and ATV FLASH, and i am streaming of time capsule via ethernet, if the file goes via iTunes then apple tv then this does not happen. Is this normal behaviour?

Thanks.