How can one go about troubleshooting occasional missing when no light or code is generated? This Sienna has 155k, and hasn't ever had a formal tune up, just good oil and air maintenance. The 3 rear plugs have never been out as they are a royal pain, but if the front 3 are an indication, they would be fine. Clean and just barely any normal wear. A complete set of coil packs might fix it, but I hate "shotgunning " parts ... Any thoughts?