So I’m on the highway driving 75 miles back to Minneapolis. About 60 miles in I notice a barely perceptible delay accelerating. Not until I was off the highway on city streets did the car begin chugging and stopping and not even getting to 20 mph. I was close to a car parts store so I limped to the front door amd and turned off the car. At that point I saw steam coming from under the hood, so I open the hood and saw the coolant reservoir coolant boiling. Being only a half a block from the mechanic shop, I stopped and was told about my radiator fans were not working. Which explains why are able to keep driving on the highway, but started having trouble when I was stopping and starting in the city.

So I’m putting out the question if anyone has had this experience for what might be good starting point to evaluate the cause. I appreciate even wild guesses!

We often see cars with both headlight bulbs burned out, and customers often ask how both bulbs can burn out at once. The answer is, they don't. One of them was out for a long time, but the driver didn't notice it until the second one failed and left them driving in the dark. That situation can happen with dual radiator fans as well, if you're unlucky enough that both of them fail without making rude bearing noises.

Thanks for the reply Vstar. That is a thinking out of the box take. Only with experience!

Here’s a curve. This morning, starting cold, I turn on the air conditioner and both fans were back on line. What could cause intermittent operation of the fans???

I'll have to look into the way they're wired, Nissan does multiple fan speeds in several different ways. If they're series-wired for low then a bad high-speed relay or blown high-speed fusible link can cause them to run fine on low but completely cut out on high. I'll check the FSM and post back later.

Okay, this is more complicated than switches and relays. I didn't know what MY your EX was, so I picked a '10 as representative. Refer to the wiring diagram on EC-511 here:
https://www.nicoclub.com/service-manual ... 0%2FEC.pdf

The EX's use a "faux e-Fan" setup where a controller drives a pair of DC fans as if they're PWM-able BLDC (3-phase) units. The ECM requests a fan speed from the IPDM via the CANbus and the IPDM sends a corresponding PWM signal from pin 97 (Violet, see EC-518) to pin 2 of the fan controller. That signal serves both fans, the controller runs them in tandem. That complicates determining the cause of both fans failing, and I'm pretty sure you'll need to catch it in the act to do so. Here are the main possibilities:
1) The Fan Relay could be dropping out. If that's the case, when the fans quit you'll see good battery voltage on pin 3 of the relay but zero or a much lower voltage on pin 5.
2) The Fan Controller ground could be bad and causing the transistors inside the controller to overheat. If that's the case, you'll see a voltage above 80mV (0.08V) on pin 7 of the controller when the fans are operating.
3) The PWM driver inside the IPDM could be failing. You should see a variable voltage on the Violet wire that corresponds to fan speed. If the signal "flatlines" at 5V or 0V when the fans quit, the IPDM is bad.
4) If the above are all ruled out, you probably have a bad Fan Controller that's quitting when it gets hot. Identifix shows those modules to be a fairly frequent failure point on all the cars that used them, G35 and FX35 also.

My dear esteemed Sir,
While what you propose as a complete decision tree with respect to deducing the cause of my intermittently working radiator fans, glaringly absent from your analysis is any discussion of the sole pin which all else rests: my head.

I do very much appreciate your earnest interest, time and effort. If, over the years to come, I am able to comprehend, and possibly take action upon, your sincere suggestion, I will again be most thankful.

No worries, you're most welcome. You can always print it out and hand it to someone handier with a voltmeter.

Here’s how it turned out. The radiator fan connectors were not secured properly from when I had a new radiator put in last fall. This is the cheapest I ever came out of that repair shop. $69.95. The loose connector explains why there was no rhyme or reason to when it would work and when it wouldn’t work. All is well that ends well. Cheers!