cannot restore iphone

i have an iphone 3gs, when i plug it into my mac the computer does not recongnize it till i open itunes and then it says i need to restore the iphone so i click restore and it gets to about 65% on the iphone and shuts down when on itunes it says verifying it. and i get an error code 6. ive changed usb ports. please help

 

i think it already is in dfu mode because i cant turn it on or nothing and the screen is always black.. also the power/sleep button is broken so i cant even get it out of dfu mode if it is