Video downloaded from youtube seems troublesome

Question

i downloaded this video from you tube and it doesnt play when i press the forward botton but the sound keeps going on for only 2 minutes from the part of the stuck vedioeven when i press the acc button it just gets forwerded 30 seconds and again stops but the sound keeps going onand when i play the movie from hte stARTING WITHOUT TOUCHING IT THE SOUND GETS FASTER (VIDEO LAG)help ?