As the title states, my 99 Miata (base, 64k miles) doesn't like the rain (or sometimes just high humidity like right before a storm).
When this happens it likes to run on 3 cylinders (I assume this is ignition related obviously).
I've changed the plugs and the wires; should I drop the $250 on a new coil too?
Run the engine at night with the hood up and check if you can see sparks? Might be also worth introducing a little additional humidity with a spray bottle.
Cap and rotor? I know my 323 hates it when moisture gets inside the cap.
Spay cap, rotor, wires, coil all with WD40. The water displacement portion of the formula should help. Apply liberally, there is nothing under your hood that can be harmed by coming in contact with WD40 (well, maybe keep it off the belts.)
^^ Yep, that's what I do with my mazda.
No cap or rotor on the Miata. Couldn't hurt to spray the coils or put dielectric grease on the terminals/wires.
im betting on a small crack in the coil post.
Replace the plug wires.
Are you sure it isn't British? 
KATYB wrote: im betting on a small crack in the coil post.
And it's better now to replace the coils and wires before it takes out the catalyst via a misfire. They are super easy to replace, and not all that expensive.
Been there, done that with my '99 Miata -- I didn't recognize the symptoms and replace the coils in time to keep my cat from getting ruined.
For what it's worth, Mazmart usually has the best prices on spark parts.
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