My 5.0 engine starts just fine, but after idling for about 30 seconds it starts to run on 7 cylinders. I replaced the complete engine wiring harness because it was decomposing. To my dismay, it still did it. I started pulling spark plug wires until I got to #1. There was no difference! I changed the plug with #2. Still #1 misfired. When I put a plug in the wire and set it on the engine it will fire...and throw a nice blue spark about 1 full inch, so the wire seems to be good. I pulled off both distributor caps and the right one had what looked like snail trail on it. I cleaned it up real good and lightly sanded the brass. I put it all back together and it still has the misfire after about 30 seconds of running. I am so perplexed. Any suggestions?