Miata A/C and Cooling issues with questions

[balactm]

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.

The fan is a weird story. It's always running, even when the car is dead cold from sitting overnight. But it does run. I've done several of the "thermoswitch tests" that I've read about on some of the miata forums, and based off the results the thermoswitch is good.

I had to replace the thermostat shortly after buying the car, and when I did the water pump, timing belt, etc, I also replaced the thermostat again. I have no idea how old the radiator is. With the low mileage on the car, just about everything looks "new-ish." 

Taylor, I live in Harrisonburg. I have my license plate mounted in the stock location, the stock air dam under the car is still in place to direct air through the radiator, and there's nothing physically blocking the radiator. At this point, the radiator and thermoswitch are the only 2 components of the cooling system outside of the engine that I haven't replaced. Should I be using a less than 50/50 coolant to water ratio? Maybe the coolant is holding too much heat? I've read about people using a 60/40, or even 70/30 split depending on where they live. I figured with how mild virginia weather was that 50/50 would be fine...

It seems to me that if the problem is at speed, then it would be indicative of the radiator not being able to pull enough heat away from the coolant while its flowing through the system. And under load (going up afton mtn) adds just enough extra heat that the radiator can't keep up.

I'm going to do the radiator this weekend since I'm also simultaneously trying to figure out why my crank is so wobbly (thanks Scott for the replacement pulley set!). To car's future is at stake pending the results of this weekend's work. If the crank is messed up (instead of just tired drive pulley), it means a rebuild on my daily driver, which is....less than ideal.