05F150XLT

I'll just have the dealer do it because I have to go there and have codes read. I have an on going misfire problem that's really starting to irritate me. It'll misfire starting at about 3200 RPM, and only when I give it more than half throttle. It's gone from cyl 8 (replaced ignition coil and spark plug) went away for awhile, then last weekend I was driving from Louisville to Valdosta and it was misfiring at 2-3000 RPM so I pulled off the interstate and had the codes read, it was cyl 2 so I had the ignition coil replaced. Truck ran the best it had since i bought it a month and a half ago. Now yesterday I took the muffler off, so naturaly I got on it, and the same problem that I had the first time, but I don't know which cylinder. Any Ideas?