I tried 2 different headphones (original iPod headphones, and PSP headphones). I use the PSP headphones (literally) all day at work, when I listen to my shoutcast station (128k, and while listening to exactly the same music, I hear nothing like what my iPod is producing).
I mostly have non-vbr MP3's, so that shouldn't be a problem. Most of the music I listen to is about 192k, so basically very common files.
I carry my iPod in my coat pocket (inside my coat that is). I sometimes take it out (during the bus trip for example) if I want to easily change the track, so then I hold it in my hands. I'll try to keep it with me in a different way, to see if it helps.
Thanks for your help guys, I was actually kinda disappointed that this brand new device already had some sort of flaw...
EDIT: I just turned on my iPod, I left it on my desk all night so it's feeling pretty cold/cool, and I tried using my , which I'm 100% sure they work perfectly, and I still hear it! So I guess this rules out the possibility if faulty headphones all the way, but also rules out overheating problems... What next?