iPod Sound Issue

I just got a 60GB video ipod, and, although it is fully charged, when I take it running with me, after about 10 minutes (almost to the dot) the left earphone has almost no sound. It does this whether it is attached to my arm, waistband, or even the treadmill, which hardly moves. It seems to do this regardless of the headphones that I use with it. What gives? It doesn't do much good to only have music for the first 10 minutes of a run.