I'm having a misfire on no 6 cylinder. It doesn't misfire on start up, and it starts up at different times (mainly in the higher gears). And there are times when it runs fine, then you can feel it when it starts misfiring again and the check engine light dings and flashes, then it'll ding and quit flashing and run OK then do it again at random times. I've changed Spark plugs, the coil on no 6, injector on no 6, all o2 sensors, and catalytic converters, cam position sensors, changed upper and lower intake gaskets, checked for broke wires going to no 6 coil and injector. It runs better (more power) after a changed the cat. Converters. The only mods I've done was adding a couple light bars. It has about 155000 miles on it. I'm not a great mechanic (more of a parts replacer), but it think that if it was a mechanical problem then it would misfire all the time, right? Any advice would be appreciated .