This sounds like a cam or crank sensor issue. I had that typed out earlier while taking a shit but i lost service when I tried to submit it and gave up.
Both of those sensors are common problems in miatas once they warm up
I originally read this thread thinking he was driving his RX7, and lol’d.
:skid
then why can i not find it anywhere on either miata.net or cr as a common issue (i did search by symptoms not cas/cps)
i know it’s prob one of those but i’m hoping its not and its fixed, i’m broke and rock auto has those listed as 300 a piece. the thing is why would it do it on a cool day and not when its been 90 out on the same trip i make every day.
for now i’m saying fuck it, until it becomes an issue again, might not be the smartest thing but living off student loans i cant really afford to replace things without knowing 100% sure its the issue, unless they are cheap and should be done anyways (why i did the fuel filter, which i think was the og one on the car lol)
ur making me almost cry in panera 
i miss that fucking car so much and wish i was driving it.
Maybe it is only nbs. If your car is a 1.8 I have a cam sensor for it
im hoping it is. mines a 1.6 short nose 90 built in 89, so it should of blown up 90 times by now too but it just keeps on trucking with 170k on it and its always been the simplest fixes before hand so i’m hoping the streak continues.