This may be a ridiculous question..but,do you use the correct ipsw during restore??i've once encounter this a long time ago when i use iphone ipsw to restore an ipod touch..the perfect example to describe is like when u want to format a PC@laptop,u have to find the correct driver or installer for your hardware e.g sound,graphic cards etc..if use a different driver, it may not function perfectly..since u said it works fine before updating,i don't think its hardware problem..better check it again..if u're sure u use the correct ipsw,then u might want to send it for warranty

It really did happen to me,maybe that time they didn't put a restriction to what kind of ipsw to install which idevice..n i didn't remember itunes have any popups like that(the mismatch ipsw with device)