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A really good diagnostic is to attach an ignition scope -- the type that's powered by 12-volts (or use an inverter) -- and monitor the ignition as you drive under various condition. Most importantly, you can watch the quality of the ignition while the misfire occurs to see if it's reflected in the spark pulses. A good scope will show a "parade" of each individual spark pulse, so you can tell if it's just one bad cylinder (and which it is) or if it occurs randomly among them all.
You'll need to snoop around to find a shop or a mechanic who has an ignition scope of this type. (I have one that I built as a kit some years ago, and it can be a lifesaver in solving these problems.)
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Don Foster (near Cape Cod, MA)
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