error code -81 is "sector number never found on a track"
I can only guess here what that means - whatever song iTunes is trying to sync next, the system is unable to locate it. Time for troubleshooting, to determine which song is causing the issue. Whether it's a faulty file, file written to a bad sector, ??, it's going to take someone a lot more knowledgeable than I am to answer.
I'd try unchecking maybe 10 songs at the point where you think the sync is stopping, try again and attempt to narrow it down.