1) try to use different usb port or first remove any other USB Device that you have connected to the computer (don’t remove usb keyboard or usb mouse ). And then try different USB Port.
2) update your itune to the latest version or download the latest version and do fresh installation.
3. Try connecting your iPod to a different USB port and restore.
4) If you are using microsoft Windows operating system, then try to update your usb driver or reinstall Windows USB drivers.
If you have iPod Mini then you need to use iPod reset utility to restore iPod.
Here's the one for Windows: http://support.apple.com/downloads/iPod_...
Here's the one for Mac: http://www.adiwebs.com/ipod-reset-utilit...