When I turn off the fireplace at the wall switch, the flame extinguishes normally, but then, about every 15-20 seconds, the fireplace will light momentarily for a split second and then go out. It then repeats over and over. It's as though a little gas is escaping through the valve, building up in the burner, lighting, then repeating. This fireplace is 24 years old and has never been serviced by anyone. It's not really used very much but the birthdays have piled up! Any advice would be appreciated.