Music doesn't play properly

My music tends to stop at random times and then continue for the rest of the song with silence. I have correctly downloaded all the songs and albums from iTunes, and when the songs do play, some just morph into other songs ie. they stop as above but start playing a completely random song instead for the rest of the song. Also, when I skip ahead eg. 30 seconds of a song, it sometimes plays a completely different song. Any help? Thanks.