Miata A/C and Cooling issues with questions

[Evan]

Do the fans work? If you're behind a truck and the temps climb the fan should turn on.

Failed thermostat in half open state restricting flow?

I hate to throw parts at problems, but if the radiator is old then put a new one in, along with a new thermostat.

Make sure to burp/bleed the coolant really well too. Miatas are more stubborn with that than some cars in my experience.