I'm guessing that you never even messed with the firmware in any way, so it's still 'stock'.
If it's a hardware issue, flashing another firmware might only exacerbate the issue.
Me...I would have flashed the latest stock JB 4.1.2 firmware. That way, it would have been easy to see if the issue was a firmware one, or hardware.
If you're not comfortable with that, take it back to the vendor. It should be still under warranty. Let them deal with it.
If you got it from TSTT though, let me warn you one time...there are no replacements 'right now', so...there's that.