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My miata has a problem and after $2,000+ in what we hoped were fixes, hasn’t been fixed.
This happens when I start my car. At first starting, the idle is a little fast but quickly drops to a slower, steady idle. Or at least it should. Sometimes when it drops to the slower idle, it will starts sputtering and shaking. Sounds as if its missing. Sometimes it settles back to normal in a few minutes. This only happens intermittently, but gets worse over time - meaning the sputtering periods start lasting longer and longer.
What we’ve replaced so far: spark plugs and wires, fuel injectors, emission sensors, catalytic converter, muffler and radiator. We’ve also checked the gas, thinking maybe water in the tank.
After each, what we hoped were fixes . . . my car will run fine for a day or two, but then starts acting up again.

If nobody has tried cleaning the Idle Air Control Valve (IAC), you may have missed a very cheap solution to the problem. Has anybody bothered to clean the IAC?

Yeah I’m with VDC. The idle air control, throttle body cleaning, or inputs to the IAC would be the first thing to look at with those symptoms. Who in the world diagnosed injectors, cat, etc? If it was the shop, find a new one.

Well, those '89 Miatas are known for that - especially once the mileage gets that high and you haven’t done any regular maintenance on it.
- Ok, yes. Sarcasm. Perhaps you could provide folks with the basic info - year, mileage, maintenance history info (rather than just parts you’ve been guessing at).

And Wow. I surely hope you haven’t just been doing all of that on WAGs. Without knowing anything I could go along with IAC as a WAG - but with the advantage that it costs almost nothing to clean it. Those are goo guesses to make.

Surely there must be a check engine light involved? If so then you need to provide the exact, specific codes in their Pxxxx format.

Chalk me up to the IAC valve as well… first thing that came to my mind when reading the OP’s post