My guess is that the hard drive might be messed up. The data as it is on the iPod's hard drive might be corrupted, or else the drive itself might be bad. What I'd recommend is wiping the iPod clean (I believe you need the iPod CD, or to download the latest iPod Updater, for this to happen) and then re-syncing the iPod with iTunes to get the songs back. If it's still not cooperating at that point, it might be time to call Apple or visit an Apple Store or Mac retailer.