Had a similar issue with a bad misfire in my '95 Passport 3.2L w/175K. I found a spot on the tube of spark plug wire #1. It was a small black and white burned spot about an inch above the rubber part that goes over the spark plug. I swapped out the wire with a used, but good wire and the trouble went away(for a month). Then had the same thing happen again about a month later on the other bank with the wire to plug #2. Only this time is was very hard to notice at first glance. Got smart and swapped out ALL of the wires and problem solved.
Hope this helps.