HOLY CoW those plugs are shot ! Thats u'r problem rite there they were arching, the white shaft part of the plug says it all. I had the same problem except my rear spark plugs/coils were submerged in water

My car drove perfectly fine under normal conditions, just when I got into moderate throttle boost it would shudder/misfire. I can see that u were prob loosing compression + arching cause of the loose plugs. I would replace those and get a re-tune just to be safe