96 Passatt has mysterious miss

I have a 1996 Passatt with the 6 cylinder gasoline engine. My wife complained that it was missing real bad. I changed all of the plugs using the Autolite Platinum plug that the parts house recomended. A few days later, my wife came back to me with the same complaint. I noticed when I removed the 3 plugs front the front of the engine that the porcelean area around the electrodes had turned a light gray color. The three plugs from the back side of the engine were still white. None of the plugs appeared to be fouled. My wife tells me that the car runs great for about 30 minutes, then starts acting up. I am thinking maybe the coil pack is going, but I don't really know.