Any single action or combo may help
Restart your iPod. Hold down the sleep and home keys until you see the silver apple. Let it reboot and try again.
Reboot your computer.
Swap out the USB port you're using
Swap out the cable if you have another one
See if iTunes is up to date. If not, update it and try again.
Uninstall and reinstall itunes.
I would start with
Run this to help if it identifies the cause
Have you tried on another computer to help determine if you have a computer or iPod problem?