I would make sure that the wires from that plug are isolated from each other, I doubt thats the issue but it might cause issues down the road.
My first guess would have been plugs, I was told to change your plugs after doing the seafoam treatment. But if it is popping it is misfiring, are you reading current misfires or history? One of them wouldn't show misfires on my 97 and the other did but I can not remember which one did.