It happens with my iPod too. ;__; But I think the shuffling happens because of that option where if you want to shuffle a song, all you have to do is shake the iPod...
Where is your I pod held when you work out? I use to keep my I pod in my bra when I worked out but it would have the same problem cause it over heated so I had to find a new way to hold it while exercising.
wtfmusicperson - I have an arm band or I use it in my hand so the cord to my ear buds won't get caught on something.