Here are some things to possibly try:
1)Make sure you are using latest iTunes.
2)Also, make sure TinyUmbrella isn't running. (It will block out the Apple servers from verifying upgrade)
3)Also, make sure you aren't running any firewall software so that iTunes won't be able to contact Apple servers for verifying.
4)Also, try using another USB port as you could have a USB port issue.
5)Also, try using another machine (Mac or Win) if you can to run the DFU restore.
6)Check your hosts file (if on Windows) to see if it's modified so that you can't connect to Apple servers for verifying.