Are you using BOSCH plugs ?, are your plugs leaking, corona discharge is ok but no streaking, zetecs are famous for this, due to the humidity inside the area where the plugs are.
Probably 50% of the bosch plugs I've used have leaked within the first few hundred miles, it's also possible the plug boots have slight tears on the inside where people don't use SILICONE grease on the inside of the boot where it goes over the plug, it should be used on zetecs.
tabetha