I've encountered a simular problem..at times; I found that by going to Tools-Options, and select downloads; and reduced the speed slider to a lower value, and did the same for Uploads, I was able to achieve playback in just about all cases. Also I would insure that The particular file extensions are enabled on your player. Good Luck

thanks to both papersky & bobinoh. Firstly I am having the problem where iTunes will not play the song ( which has been sent there ) & second I will try the suggestions & see if I can get any results. Will let you know!