Try a new radiator cap.
Squeeze the lower hose while it's running at operating temp to see if it might be collapsing. Have someone rev it while you squeeze. Feel for soft spots. (I'd replace it)
Other than that, next guess would be water pump.
Had you done ANYTHING to the coolant system prior to this happening? (like a flush and coolant replacement?) If so, it could be an air bubble. You can leave a gap at the top of the hose where it attaches to the radiator to "burp" the air out (again, at operating temp, so the thermostat is opening, allowing full flow).