missing under WOT
 

My Spec Miata, 99, starts to miss at about 6500 rpm, in any gear, at WOT. Not all the time, but often enough.
It may be heat related, OK early in a run, then developing as the engine bay gets hotter, but it is not always that clear.

I replaced the coil pack with another used one, and it stopped for two weekends, But now it is back.
The engine is freshly rebuilt. The fuel filter was changed at the start of last season.

What are the other possible culprits, are there some well known ones?

Stock coils are very susceptible to heat. My car was same way (not raced, just street driven) until I went to COPs. Replacing coils helped a little, but always missed when car got hot.

I went to COP's just for insurance. I did not want to have to purchase a new, stock coil, when one went bad; and THEN go to COP's. Toyota COP's have plenty of spark for my power level.

You may consider the NGK Iridium spark plugs as well.
DNM

Spec Miata - stock coils only, no COPS
Currently running the Ir plugs

Is there a way to get more air from under the hood? Like hood venting? Not sure what's allowed.

Coils are a known weakness, but crank and cam sensors can have heat related failures.

Trying some additional heat shielding and maybe yet another set of coils may help.

SM rules are very strict, no real way to do much about the cooling in a 99.

I can look at the crank angle sensor wiring, the cam angle sensor wiring, and another coil pack. But it feels like throwing parts at the problem. However, there may be no choice but to take that approach.

I just thought it was worth asking about the main culprits, so I have an idea about which parts to throw at it, and don't miss some.

My guess, bad coil pack. Buy a new one.

I think Ballenger Motorsport makes a cam sensor specifically for this purpose. i don't know 100% if this is your issue or not but if I were running a spec miata I would likely be running this too.

https://www.bmotorsports.com/shop/pr...oducts_id/4307

Do rules allow for a NB2 coil pack? They are quite more reliable than NB1 ones.

What Harry said

I believe that. Sounds like a good idea if allowed.
I will have to check the rules, but I know the general rule is no updating, no backdating between the different years, unless expressly allowed, like some differential swaps are allowed.

Buy some cheap copper plugs and gap em on the tighter side of spec. They tend to run cooler that IR or Platinum plugs. You can also try dropping one heat range as well since fouling is less likely in your situation.

I know it sounds too simple but I would definitely start there.. Some well respected tuners over the years kept pointing me to copper on my big power builds. Misfires was something I always struggled with on IK22's and IK21's. Once I swapped to copper (Bosch or NGK) my problems were solved.

If allowed by the rules, replace your cam and crank angle connectors with DTM connectors (that's the meat of what Ballenger is selling). The OEM sensor are not meant for long term motorsport use. Make sure the cam angle sensor wires are strain relieved.

I experienced the same with my Spec Miata, it would miss at 6500. CAM Sensor, the one at the front of the valve cover.

Thanks to everyone for their input.
When I solve it I will report back.

Well, thought I would complete the discussion.
Replaced coils, not so bad but still there.
Replaced cam angle sensor with the fancy pig-tailed version referenced above, improved, but once in a while a hiccup.
Replaced spark plugs, problem solved.

Sadly, then had to replace engine, due to thrust bearing failure.
Went with new coils and cam angle sensor, but used old dirty plugs during break in.
Once in a while a miss around 6500.
Replaced plugs, seems to be gone.
At least the new engine pulls nicely.