When running such cold plugs you need to make sure the engine is not to rich and floods its plug,
If you keep starting the engine and not warming the plugs you will kill them, I recomend when doing this is to hold the revs to 2.5k until the engine clears itself after 30sec then they are normaly fine to starting the car,
If misfiring under boost you will need to close the gaps to the correct range for your coil system,
Running 31's as such low power is IMO not the correct way to run this plug.

Mark