I have a 2008 GMC Sierra 1500 5.3 that recently started having misfire issues and around the same times I lost power and as the truck warms up it gets a loud exhaust noise , Im assuming one of the Cat converters is failing or both and that is the exhaust noise from the doughnut gasket burning out . I am wondering if this is the reasoning for the misfires . Or do I have other issues . I have 8 of the best plugs recommended by GM for the truck ready to be installed . Any help on what to look for would be great.