IPhone4 in recovery mode, restore stops half way through with an error message.

Hi, please help if you can. I have an iPhone 4 running the latest IOS which for some reason hung on start up with the Apple logo. It would get out of the loop, so got it into Recovery Mode, connected up for restore fine. This happens fine, then half way through the restore there is an error message, saying the restore cant be completed. I've tried it on three machines, different cables, different USB ports, and am now stuck.

Its unloced but not jail broken.

Thanks go your help.

Nick