Why does Zune only play 30 seconds of a song?

I have a zune and the program associated with it. I have an external hard drive connected to my computer which has all my music on it. I do not have demos of songs, I have full songs. Every once in a while, Zune will get mad or something and play only 30 seconds of every song. Its aggravating and annoying but I dont know how to stop it. It doesnt always do this just randomly. Anyone know how I can stop this? Suggestions? I dumped it completely off my computer and reinstalled it but it is still doing it.