Bet ya 5 bux that 1 or more of your exhaust valve is burnt, causing leaks during combustion phase.
My take would still be that your spark plug #3 (from left to right) has a broken ceramic tip, that tip might have caused some damage....
At this moment, no point make any more guesses (even from myself). I would simply get a mech to pull the head for inspection if I were you...
My 2c's worth.