Mini Problems.

Hi, I've been having problems with my mini lately. For some reason, my iPod only playes music purchased from the music store sometimes.

When It won't play my music It skips to the next song. If I go to my purchased playlist it skips through all the songs and then goes to the main menu when It skips through all the music.

The only way I can get it to play the music that I purchased is if I plug it into the computer and have it update my music.

So, what I'm asking is what should I do. I have over 100 dollars worth of purchased music and I wan't to listen to them without a computer (the point of an ipod). When this happened the first time I reset it and It worked fine, but today It didn't work.

Thanks.
CG