problem in playing flv and mp4 downloaded from youtube

hello. recently as i download flv and mp4 files from youtube my playes such as KM player, VLC media player , jetaudio, real player, windows media player plays them skipped it means that softwares jumps to next seconds and a 2 minute file is played in 30 seconds.
this problem doesn't happen to my flv and mpeg files download from YOUTUBE.