I have a 1990 5.0 mustang and it was running great but then as I was on the gas in 5th, it started to misfire... The headers on cylinders 6 & 7 were 130 degrees, and the rest were all 220, so I pulled the plugs and sure enough, 6&7 were both black and the rest were still white. I replaced all the plugs and still got the misfire, but I checked and the plugs are sparking. It also started to knock pretty loudly from the bottom end, but only if I rev it up quickly. Any ideas or suggestions as to where to go from here?