So after further analysis, and although the radiator fans seemed to run fine, it turns out that the fan motors were no longer able to reach a fast-enough operating speed to sufficiently cool off the system when the weather is very hot and AC is on. Inside of the main fan motor the carbon dust is thick and the carbon brushes are well worn out.
I replaced the whole fan assembly ($75 aftermarket part)
Later replaced key system parts as preventative measure ($200 for all Nissan OEM parts)
- Thermostat, with housing and gasket
Water pump
Lower radiator hose
Radiator cap
Nissan long life blue coolant
MAF sensor
Air filter (FRAM)
All in all, with the new radiator, it's a $1000 overhaul job (parts and labor) but the elegant lady is back in good condition.