Just a thought, have you pulled the spark plugs to look at them, it's possible one has a cracked insulator that could cause a short, for a couple bucks apiece it prolly wouldn't hurt to replace them anyhow... If its anything like my camaro you have to get under the car to replace the back two, when I first got my car it was only running on six of eight cylinders because whomever owned the car before me did a tune up and was too lazy to change the back two plugs