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Do you have a thermostat installed?
No.
Could be your motometer. When the engine is shut down the temp should go up not down. With the water no longer being circulated the heat collects in the high point of the radiator and should send the temp up.
Yes, the temperature on the MotoMeter does climb when you stop. It quickly falls back into the normal range when you're back on the road after the stop.
Next time it does this feel the radiator, is it really hot or does it feel normal?
I just got back from a 350 mile trip, I'll try this out when I'm ready to cram myself back in the cab for an hour. 
Do you have any out flow at the tube? If it was getting that hot you should have boil over.
Yes.
Is it foaming? I have found with my Model T, some water pump grease has caused a bit of foaming and if that foam touches the motormeter causes the temp to go up on it.
That is plausible. I have noticed that sometimes on inclines the temperature will spike & then quickly decline, and I was wondering if it was because the water was temporarily touching the MotoMeter itself.
Motometers read the air temp above the water not the actual water temp.
Yep. Out of curiosity, does anyone know what the air temperature itself is in the normal range when the water is normal operating temp?