itunes says "itunes cannot read the contents of the ipod "xxx"...

every 5 - 10 syncs between itunes and ipod (classic and nano, newest ones) it gives this error message:

"itunes cannot read the contents of the ipod "my ipod name". go to the summary tab in ipod preferences and click restore to restore this ipod to factory settings."

why does it do this to either ipod? how can i fix this without restoring my ipods all the time?

itunes software is up to date! please help!