What is your temp difference between the top and bottom hose? And take temp readings all over the radiator to make sure there are no hot spots. Check your bypass hole in the water pump housing to make sure it is not too big. Check both sides of the water pump housing and both heads as it comes up to temp to see if they are even or if one gets hotter quicker. Maybe change the water pump pulley to speed up the water flow. I would pull the thermostat and see what happens.

The old adage that slow vehicle speed overheating is usually air flow related, while road speed overheating is typically a water flow issue doesn't seem to apply given your description.

20* drop top to bottom of the radiator in this application sound about right to everyone?