most likely it's humidity causing the arcing. clean and grease the cap anyway. after you clean it inspect it for any carbon tracks (looks like jagged black lines across the mating surface of the rubber cup, or rarely, straight black lines). if that's the case, the rubber cap is beginning to break down and, although you can clean the carbon out and restore the insulating properties of the cup, if it's beginning to break down it should be replaced. arcing, heat, and corona discharge cause the rubber to de-polymerize along the track, and although you may get all of the carbon out, the rubber in the immediate vicinity of the track has partially decomposed, and may be likely to form more carbon tracks in the future. silicone rubber in this condition will be crumbly, and may be brownish or a lighter grey than the rubber around it.. silicone rubber is a very good insulator with excellent high temperature stability, but it has it's limits...