Hello,
I have a 2002 ls1 from a firebird in my 65 impala. Car runs and drives great. After winter, the first time I try to run the car the past three years I get s p0306 code. Misfire in cyl 6. I trouble shoot everything (compression, injector, coil...) and get a bad plug. Last two years the plug looked brand new with no problems. this year the plug is dark 3/4 of the way around the insulator but clean on 1/4. The tip is shiny on the electrode and the arm is clean and you can see a distinct dot where the spark jumps directly to the arm.

I am confused to why this problem with the same plug and only after sitting all winter. I do start the engine every month and drive the car around the block and let coolant get up to 200. Fans are programmed for 200, car rarely gets above that.

Any suggestions appreciated.
I did try ngk plugs and 3/8 misfired so I went back to the ac delco ones. This was over three years ago.