Most new t-stats have either a check valve or even a small hole (bleed hole) drilled into it so a little water (and air) will pass even if the t-stat is closed.
If you drain the coolant, there is air in the block and the t-stat is closed (as the motor is 'cold'). As you fill the radiator/block the water rises, but the seal on the t-stat will not allow air to escape as the water rises. So you have an air bubble in the block. Since the motor is normally tilted up a few degrees it compounds the problem of the air bubble right at the water pump/t-stat.
If you drained 2 gallons, you should have put 2 gallons back in, otherwise there is air in the system.
That's how you get air bubbles in the system that can do what you are seeing.
The t-stat is right on the top of the motor. Drain a few qts of coolant, look at your t-stat and see if you have a check valve/hole if not, take it out and simply drill one.
T-stat with the bleed hole Also, if you run the motor with the radiator cap off after the temp reaches 180 of so the t-stat should be open and you should be able to see the water come from the top hose. Look for bubbles and moving liquid. Its easier to tell whats going on if you don't fill the radiator all the way, that way when the t-stat open you should see coolant rush into the top tank via the water pump.
