Shoutbox

Try defragmenting your hard disk in safe mode, then take it out of safe mode and see if it happens...That may help enough
But I really need to know the rough specs of your comp (processor mainly) to guess why it's happening, but it's probably because your computer is running slow.
Also try a low system requirement media player, such as media player classic, or a really old version of windows media player. Even BSPlayer may do it. Winamp on the classic skin may also do it...