Are you sure you don't have an air bubble trapped in the system? Sometimes when you get a project done and you fill the system up, you normally fill the radiator and then as it drains into the block, you keep filling to get it full. But the coolant is pushing an air bubble into the system and can stay there until it breaks. So the motor gets real hot real fast. The way you solve that is to just cycle the motor, let it get hot, then let it cool down. eventually the bubble wil get pushed out and the coolant can then circulet much better.