music suddenly stops mid-song

Lately, my iPod nano (gen 4) will stop in the middle of a song. I'll pick it up and advance to the next song and it will happen again. But here are the really odd things: first, only music stops, not pod casts or audio books; and second, so far this has happened only in my car (plugged into the audio port), not when it's plugged into exercise equipment. Does anyone have any ideas as to what might be going on? Is the iPod nearing its end date?