My ipod keeps shutting down when I'm on exercise equipment

I've had three iPod shuffles shut down this week. It's very strange. One was a brand new 4th generation. I was on an elliptical machine at the gym and it stopped playing. I never got it started again and it's not charging or recognized by my computer. I've done everything it says online and it is just dead. Same thing happened to another 4th generation that is about a year old. But it has since charged and appears to work, but have not used it on exercise yet. Then today, I was using an older 2nd generation iPod and it shut down 25 or so minutes into my exercise on this same piece of equipment. Just went dead. Could there be some sort of electrical thing going on? Am I getting sweat into them? I have the clipped to my shorts and wonder if maybe my body is some how disabling them?