I've been thinking about this do you have good hot heat this would indicate the water pump is good. It would also indicate the air is out of the system.
The only other idea I have is trash in the motor I use a water hose with both radiator hoses and the thermostat removed then I use rags around the water hose stuffed in to the top hose and flush until it runs clear. You indicate springs in both hoses so they don't collapse.