Here's where mine gets with no air flow at all just idling in my garage.  240Z with a larger 280Z radiator and full shroud, half antifreeze with the other half distilled water.

Sounds like a sticky thermostat.  Mine used to fluctuate, even though it tested okay in hot water.  A new Nissan unit gave a steady temperature reading.

You can't beat Nissan thermostats!  They are the best in my experience.