From what I have read, i think you have changed your valve cover gasket on your 4 cylinder 1998 Frontier before. I am about to change mine. My question to you is: The gasket is a "all in one" gasket with the 4 cam plugs and the complete gasket is rubber. Did you use any RTV on this gasket any where on it OR on the whole gasket? Normally, on a rubber gasket you are not supposed to use any thing but I want to do this once.