sync problems

As a test to see if it makes a difference, select the iPod and go to its Summary screen. Check the box for Enable disk use (if it's not already) and Apply the change.

Try syncing again to see if there is improvement.

I Google "error 1303 ipod" and it appears this error is a Windows error that happens when there is a problem accessing the program's stored data, I guess in this case iTunes accessing the iPod's data storage. This website has good info, with some suggestions

http://www.error1303.com

Since iTunes is able to start syncing, and it work properly some of the time, it seems like there are occasional problems with the USB connection, which makes the iPod's storage unaccessible momentarily, and you get that error message. So for short periods, such as when doing a manual load (one or a few items at a time), it's OK. When the transfer time is longer, such as when automatically syncing a lot of new stuff, the problem is more likely to occur.

In addition to other advice you may find online about this error, as another test, I would try shutting down and disconnecting all USB devices (except keyboard and mouse if normally used), including any hub. Start up the computer, and do this to reset the iPod

http://support.apple.com/kb/HT1320

Run iTunes and connect only the iPod to a direct USB port. (If your iTunes media data is stored on an external drive, you'll need to connect that too.)

See if there is any improvement when syncing. If it works better, there may be some conflict (or interference) between USB devices. You will need to use a process of elimination to see if one of your other USB devices (or a combination of devices) is somehow interfering with the iPod's USB connection during a sync.