1. Try another USB cord or USB cable.
2. Uninstall and reinstall iTunes for a try, this is the method that I got from Apple technical support.
3. Reset or restore your iPod for a try, this can fix many general iPod problems.
4. You may try this iPod transfer, it's a nice iTunes alternative that can help you upload music and video to iPod, I always use it to sync my iPod when my iTunes cant detect my iPod, it's easy to use and works pretty well
http://www.youtubetoipod.biz/iphone-ipod-to-co mputer-transfer.html