I know the iPods have been talked about for their skipping/freezing, but I have never had that problem. Course I'm jogging, not running, and only for a few miles.
If I can make one suggestion before you buy another player-
Most CD's, MD's, or mp3 players have a buffer. That buffer keeps it from skipping (and protentially freezing). Certainly skipping can occur for other reasons (the hard drive bouncing around inside the iPod. etc.), but for a second lets talk about that. I use an old spandex Sony MD waist belt (no idea what the model is). The iPod is a little snug in it but when I wear it the iPod ends up around the small of my back.
I suspect keeping the ipod there decreases the amount of horizontal and rotational movement and probably the vertical bouncing (impacted by other causes like leg muscles, etc.), though it certainly won't help with the fact that your body moves up and down when impacting with each step.
Hey, there's a good chance this will have absolutely no effect on your problem, but it's worth checking out before spending more cash.
Ric
Oops, see this started a long time ago. Maybe it's too late.