Why is engine missing when cold and damp, smooths out when warm?

Choke not working properly

check the connections of your coils, and the resistance of your plug wires. miatas are notorious for eating plug wires, sometimes a lead wire from the coils can ground out on the cylender head or valve cover, disassemble and clean. reassemble with dielectric grease. perhaps new sparkplugs are also in order.. also try cleaning and greasing the grounds in the car. "mazda" or FORD cheaped out on most wiring in these