STEP 1: remove the small hose from your coolant reservoir, hold it over the opening of the coolant jar. With the engine running, you should have flow. No flow= bad water pump or a restriction in the system. Yes flow= step 2.
STEP 2: In the morning, engine cold... take the cap off, start the engine. If coolant immediately rushes out of the bottle, you've got yourself combustion gases in the CS... bad head gasket, basically. If this doesn't happen, go to step 3.
STEP 3: Verify your heater blows hot air when at operating temp. Then, engage the A/C and put your hand downstream of the air that the cooling fans are blowing. Does it feel hot like the heater, or cool? If it's cool, put a thermostat in it. And while you're there, replace the housing and also the crossover pipe. They'll eventually break.