happens gradually at about 5weeks and continually gets worse until i have to replace them. I have replaced all plug wires. The electrode end of spark plugs look good. It arcs from the base where the porcelain insulator goes into the metal. They dont all arc at the same time . It is erratic and the car idles fine with an occasion mis. The hotter the engine gets the worse it gets especially under load. I had it put on computor and it says that i have bad spark plugs but it doesnt say why they keep going bad.