Driving the Grand Prix and upon slowly pulling out of a parking stop I thought I heard some bad knock in the engine bay. Didn't think anything of it and after a hour drive hour via interstate I start noticing a slight misfire at idle. Scan and it turns out #2 & #4 are missing. Spray brake cleaner all around and can't find a leak (still pulling 21"mg at idle in park). Pull the plugs and both insulators are cracked and slipped over the electrodes. Replace the plugs and now I only see the misfire on #2. Had a bunch of parts left over from when I built the car and tried wires, coil, computer under coils, and injectors with no help. What are my next steps? Lifter? As I originally broke #2 & #4 simultaneously, could it be the head gasket between the two cylinders?
Thanks,
Matt