Make sure you are actually getting it into DFU mode. A VERY good percentage of the time people don't actually get it in DFU mode.
DFU Mode=iTunes detects a phone in recovery mode. Screen is completely BLACK.
Restore Mode=iTunes detects a phone in recovery mode. Screen has stuff on it like "Connect to iTunes"
If the phone is actually in DFU mode, restoring the stock firmware will completely wipe the phone. If the problem persists through a DFU-mode restore, then you probably have a hardware problem (assuming you or a previous owner hasn't done something silly like install the iPad baseband)