So you ran injector cleaner through and checked the blue
sparks produced by the spark plugs.

Maybe someone else has something else specific to suggest.
I can only look at the Enthusiast Manual:

Disconnect ISC wiring connector while idling and check that
clicking is heard from valve.
Disconnect oxygen sensor wiring.  If fault improves, check sensor.
Stuck air flow meter.
Check ignition timing.
Put a screwdriver against each injector and listen
for operating clicks.
Check fuel pressure.
Defect ECU.
Low engine compression due to wear or damage.
Air leaks or obstructions in intake system.
etc.

Also check the crankshaft nose problem on miata.net

Leon