2006 Mark LT with 100000 on it. Got the P0305. Figured I'd start with the "cheapest" possibilities, so I replaced the spark plugs, coils and fuel filter. I was super stressed about the plugs as you can all figure out why. But I was surprised when they came out easily. I'm guessing a previous owner replaced them because they all had anti seize. It ran a little better, but when I passed a car on the freeway, it downshifted, and misfired again in the 5th cylinder at about 4-5k RPMs. I replaced the injectors. Ran smoothly again. But after a few hundred miles, I floored it going up an on-ramp, and sure enough, it misfired again. Just for the heck of it, I put some sea foam in the tank, expecting no results, and that's what I got. Still misfires almost every time it downshifts and RPMs shoot up to 4-5k for more than about 5 seconds. Any ideas?