In support of what Arthur is saying, I have had a similar experience.

A single-plug misfire, replaced plugs, no change. Ordered new wire set and 1 new coil, didn't need the coil, it was one of the wires.

Seems odd that a bad wire (or one bad plug connector) would affect only one plug when there are two in series, but it is exactly how it worked for me. It ran as yours does: idled on 5cylinders, but easily made highway speed on 5, just a vibration at lower RPM, but never seemed to fire on all 6 (the engine management will cut injection to the misfiring cylinder so it doesn't get a second chance anyway, ... does save the CATs from damage though).

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Gone to the dark side

- Jeff