I have a 1997 GMC (5.7) with a miss that I can't figure out. A friend of mine hooked his engine computer up and it says cylinder misfire. I could tell that by the way it runs. Changed the spark plugs checked the cap and rotor (look good) and it still runs terrible. It sounds like it's running on 6 or 7 cylinders. The engine shakes at all RPM's. Any suggestions on what I should check next?