In installing the theromstats, did you "burb" the air out of the system? The system can "air lock" towards the fire wall. I find that filling the system full with the front of the car higher than the rear helps to get coolant back in to these places. Have the car running and keep filling the radiator to the top. There are bleed caps in the system on the water hose metal connectors that can be carefully opened until water comes out - then quickly close. You can also obtain small valves to which you can add plastic lines. I run the lines into the coolant fill opening. Run and fill with water with valves open, watching for air bubbles. When there are no more air bubbles, close valves and radiator cap. System should now be free of air. If air lock was the problem, it should now be cured.