My Equinox has developd an engine misfire causing the engine light to come on a times that is not showing up as a code in the computer when the diagnostics are run on it. Is a particular problem on very cold mornings and will only last for up to 5 seconds. Shudders and shakes very badly upon initially driving then goes away. Recently had spark plugs & wires replaced (bottom of #1 plug was burned away) and then catalytic converter had to be replaced. Have gone to Chevy dealer and 3 other mechanics but no one can figure it out. Fuel injectors have been cleaned twice. Last mechanic suggested a new coil but haven't replaced it yet. Anybody have any thoughts?