Ipod Nano Skips past certain songs.

Hi, My ipod nano has been skipping certain songs lately. These songs were downloaded, not from itunes, but from another location. I have reset my ipod(Holding menu + center button), restore to default factory settings, and double checked the ipod software and made sure it is up to date. I still do not understand why it skips certain songs. All songs that have been bought from Itunes work fine, however only the one downloaded have an issue. I am angry at this issue and I do not know what to do. Please help me.

-Thanks