ipod shuffle randomly disconnects

i get the message that the disk volume was unmounted in a dirty way, although I didn't even touch it.

the behaviour is quite non-deterministic, but especially when copying many files on the ipod it is more likely to do this. sometimes after disconnecting it physically, both usb plugs don't work any more, even for a mouse.

kernel[0]: disk1s1: media is not present.
kernel[0]: disk1s1: I/O error.
(both many times)

what is this problem due? hardware? software? the ipod? powerbook?

also note that until recently everything worked well.

thanks for any hints