Strange volume issues

Hi there I seem to have an occasional problem with the 60gb (MA147FB) running version 1.3. when I have it plugged into the aux port in the car via 3.5mm jack, I will be driving away listening to music and then all of a sudden the volume of the track will be altered "as if the click wheel is being activated". I think I should also point out it is in an iskin evo3. Could it be a vibration from the car causing the rubber to vibrate over the wheel and alter the volume?
Or is it possessed???