You probably have an AIR BUBBLE in the system.
I have my patented method to remedy this:
Make sure that the coolant level is topped up just a bit overfilled.
Get a set of 6 to 8 inch ramps. Drive up onto them. With a cool engine, start the engine in neutral. Make sure the hood is open. Crack the air-bleeder screw and let the engine come up to temp. You will all of a sudden see coolant and air start to bubble out once the thermostat opens. Let it keep bubbling until you see no more air bubbles mixed in with the coolant. Now close air-bleeder screw tightly.
This nose- in- the- air attitude make the air bubble slide up and out the front of the head and engine allowing the coolant to completely maintain contact with the hot metal instead of air.
Thats a good thing.
If you do this proceedure on a flat surface..... you'll keep your air bubbles trapped.
Not a good thing.

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