I assume you're streaming these files. I too have had issues with mp4. When you comment that the smaller files have more problems, that may point to the compression types that are being used. The previous poster suggesting Tversity is correct, this helps considerably. As an alternative, try PS3 Media Server. I find this a better solution, as it can handle more formats. With MP4, I find I sometimes have to specify a different transcode engine in order for it to play.