The heat range of spark plugs means how slow or fast the plug tip dissipates combustion heat. Alot of it has to do with insulator shape and size. Cold spark plugs dissipate combustion heat very quickly to avoid pre ignition from having to high combustion temps, maybe what the OP was experiencing. Hot range plugs remove combustion heat slower, typically small engines saws, mowers etc. Use higher heat ranges bc the engine doesnt build high heat on its own so the hot plug is used to keep heat in the combustion chamber longer to avoid carbon fouling. Cold range plugs are used in high heat engines hi performance or race bc they can build high heat rapidly so the plugs needs to get rid of the heat faster to avoid pre ignition