Pick a spot and start eliminating things.
IF ...you think it's the coils, then maybe start at the end of the chain for the secondary side of things.
Start with each sparkplug and work backwards.
Is the plug good? Is it clean and gapped? Find one of the MANY color charts and read them.
Is the plug boot good? Is there corrossion inside the boot where the spring clip is on the plug side or a corroded screw on the wire side? Can the spring clip be removed so that the internal resistor can be checked? If so, do it. Otherwise do a resistance test on the boots, all of them. There are ohm ratings that need to be verified. 5K ohms I beleive.
Are the secondary wires showing clean metal? Corrossion? Gunk in general? Maybe trim them back 1/4" at a time until there is fresh metal for the screw in the plug boot to grab hold of on that side. Same on the coil side of the wire.
Check the screw in the coil for the same corrossiveness. Check it for physical flaws of any kind, such as hairline cracks.
Check total resistance of the secondary side for each cylinder. Check when cold and when at operating temp having the fan cycle at least a few times to warm the coils as best you can. If there's a weakness due to temperature, maybe it'll show.
If disassembling any of the above, reassemble with ignition grease is my suggestion. Keeping moisture out of any connection is paramount in secondary ignition preventative maintenance ...in my opinion.
Knock-out the possible secondary failure points and, if they check good at the end, move to the primary side.
It is the secondary side maintenance that will show a bad coil because... what goes in must come out.
Checking primary side function is a look at ignition triggering and amplification to the coil. There could be a bad wiring issue making the coil do what it may or may not be doing, but that is not indicative of a bad coil. Bad primary wiring has nothing to do with the coil being defective.
This is how I approach my ignition headache stuffs...
Having access to a ignition scope, or knowing someone that has one AND KNOWS HOW TO USE IT, can save lots of everything. Just swapping parts and hoping that's it doesn't have to happen if you spend the asking here, then trying the suggestion, and doing it again until the problem is found and corrected.
