I donno, i'd bet a chipped piston. I knew a local once who kept getting his gaps close on his plugs. It would happen every once in a while, he go WOT (or into boost) it would happen and the car would misfire suddenly. He kept plugs on him, he'd just pull over, change it or re gap the old plug, and go on for several days. Did this for months until finally completely blowing up a piston. If I remember correctly, when he yanked the heads, there were 2 or 3 other pistons chipped as well. Its been a long time.
Anyway, I saw a couple of his plugs and believe it or not, they had no noticable marks on them like metal hitting them. They just looked closed like someone didn't gap them at all and threw them in. He ended up regapping some and sticking them back in.
I always figured the reason the plugs never had marks on them was because of all the heat and the slight hitting of the chunks of pistons bent it easily, thats the only guess I have.
Get a boroscope or yank the heads. I think you need to take a look inside the motor a little closer. BTW, if you do get access to a boroscope, check ALL the cylinders. Pieces have been know to move from cylinder to cylinder. You shouldn't be going through plugs like that.
