I had a similar problem after the 100K mile plug change. After numerous code readings, dead ends, and about $1400, it was cured by two things: (a) autolite double-platinum plugs (took care of part of the problem) and (b) 9mm plug wires (I had previously replaced the wires with dealer-supplied new wires, which did zip).
The forum did supply the info about the plugs. Plus, you get your choice of colors!
