Why are the songs I purchased on my iPod not playing on my iPod?

I have a 3rd generation iPod Touch. Often I'll download songs directly to my iPod when I don't have to time to sync with a computer. When I first get the songs, they play normally. Then after a couple of days, it gets weird. They'll play for five seconds and then start over, not play at all and force me back to my playlist, or they'll play without any actual sound. These songs normally play on my computer. Why is this happening and how can I fix it?