Songs suddenly stop or skip in iTunes (randomly)?

First off, this issue isn't just in iTunes, I also use j-river Media Center to play music too.

I have all my music files ranging from MP3, to m4a and even some FLAC (which I know iTunes does not support) stored on an external HDD. I usually do the ol drag/drop and let the media players do the rest.

But I am often irritated when playing my favorite songs it will just quit in the middle and start the next song. Or, skip and start back up again. I have tested to see if this is a bad file by replaying, but the problem seems to be inconsistent to the place of the song it happens. Also, if I play the songs in Windows media player they seem to work fine (that I have seen so far). I use iTunes and J-river over Media Player for obvious reasons so please spare they "just use that" answers.

I have also noticed these issues happen on my iPod as well.