My 99 miata with 65k miles on it is still stumbling at idle; ONLY when it's raining/going to rain outside.
I've replaced the plugs, coils, wires (all new from mazmart) and it's still acting funny when it's raining/going to rain/just rained outside. It seems to run fine and idle just fine when it's dry outside.
any ideas? 
Hmm so either triggered by temp or humidity...no error codes on the ECU? Nothing that suggests it's running lean under the same conditions?
i don't think it's temp; maybe humidity?
no error codes, I don't think it's running lean, but i guess it's possible?
It's not pinging; just stumbling, like it's not getting fuel/spark?
If you've changed all of the ignition totally, how about clean the air meter.
You can get the more gentle cleaner at any parts store- it's possible that the humidity is messing with the flow meter if it has anything on it...
Or have the fuel system cleaned out on it...
Miata MAFs are pretty robust. Won't hurt to clean it, but I'm thinking there's water getting in somewhere. Next time it's stumbling, try hitting the connectors on the ignition system with WD-40.
Maybe your Miata is originally from California and just can't deal with weather.
I'll try that Keith. Thanks!
It's an NJ (California emissions) car that was converted (the right way) to a fed spec car. the issue has been around since before it was converted though.
On my way into work today, I hit a small bump and the car just died... no power, it won't restart... I checked all the ignition components and it's acting like it's not getting fuel now. It almost starts, but doesn't.
I'm going to be stuck at work unless i can fix this in the lot.
I've seen this issue once before, going around a sharp left turn, the car would lose power, then be fine again a few seconds later.
I'm guessing fuel pump, but it sounds like it's providing pressure?
Sounds like you've got a wire loose. Could be a ground, could be power to something like the fuel pump or coils or ECU. Or a bad main relay. You're going to have to find out what's not getting power.
i can hear the fuel pump kicking on when i turn the key, and i can smell gas, so i've ruled out the fuel part. it has to be ignition. the main relay is the large green one under the hood in the relay box on the passenger side, right?
i've swapped around the 30a relays in there to no avail (to eliminate one of those as the issue)... i'm guessing a ground or wire loose now as well.
any way to troubleshoot spark? can i yank just a plug wire and see if it's arcing? (i have minimal tools here at work, couldn't pull a whole plug without someone bringing me my toolbox from 20+ minutes away)
If the main relay is down, you won't get fuel or the pump IIRC.
You might be able to look for arcing on a plug wire. I'd rather do it with a plug, but you work with what you have. If you've definitely got fuel, spend your time poking around near the coils. Wiggle things.
If you have to troubleshoot spark with no tools, pull the plug wire just away from the distributor cap and see if you can see a spark jumping across. KEEP YOUR FINGERS OUT OF THE PATH OF THE SPARK and put it back immediately if you do see a spark. You can do it with all four to be sure.
I had a similar issue once, the connectors had rusted in the distributor's power filter capacitor.
Miatas have distributorless ignition, but the concept is the same. No need to pull all four wires, 1-4 and 2-3 are paired. If you lose one, you'll lose both.
So the spark seems to be working okay, as does air... that just leaves fuel; correct? I've checked all relays and fuses, all seem to be okay... i've even jumped the fuel pump via the diagnostic connector to no avail... any other ideas? i'm thinking it's a fuel pump/fuel filter... although it died VERY suddenly.
Check the electrical connections between the ECU and the injectors, and it wouldn't hurt to trace the fuel itself from the tank up to the fuel rail. If youve got good air, good spark, you can hear the fuel pump running, and you can smell fuel, those sound like the possible culprits, although, fair warning, I've never tried diagnosing a fuel system of any kind, so grain of salt and all...
i went over a little dip coming into the garage at work yesterday, like 20 feet before the motor cut out... is it possible the fuel filter ate something that the dip dislodged?
Sometimes we have fuel, sometimes we don't 
I think you're going to need more tools.
It's unlikely you picked up something in the tank that immediately and completely blocked the filter. Unless you're storing plastic bags in the tank.
Can you expand on that? if i get it towed back to my place am i looking at a fuel pump?
I don't know what you're looking at. It's still unclear as to whether you're getting proper fuel pressure or not, that would be the first thing I'd check. If not, then the fuel pump is a big possibility. But given the failure mode - hit a bump, immediate failure as opposed to sputtering to a stop - and the previous moisture related problems, I'm still thinking it's electrical.
Had a fun time with my Mustang years ago.
Had an intermittant no-start condition. (wasnt getting fuel)
Replaced the fuel pump and impact shutoff switch.
It happened again, it ended up being a 5cent connector going bad
check the impact shutoff if a miata has one, then go through and check all connections in the fuel system. Clean and apply dielectric grease
Don't think there's an impact shutoff on the NA and NB miatas, i've gone through some of the connections, but it sounds more and more like i need to get it towed.
No impact shutoff.
If a Miata had an impact shutoff switch, many SpecMiatas would be stalled out on track if they weren't removed for racing 
okay, so the fuel pump lines (red wire with blue stripe) are not getting power, what does that mean?
Put a jumper between the F/P and GND pins in the diagnostic box. Do they get power then? If not, then either you've got a bad fuel pump relay or you're not getting power to the fuel pump relay. A bad main relay would cause the latter (it's what dumped me out of the Targa last year) but then you shouldn't have spark.
Diagnostic box pins.
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