1997 Chevy Monte Carlo Does not warm up at idle

Recently replaced water pump. Now when car is started in the morning, the heater does not get warm. When you drive it the temp gauge will go up way above normal and then drop back down to below operating temp then slowly go back up to operating temp and the heater will warm up. The gauge will then float up and down and finally stabilize and heater works fine. I have noticed that when I stop and check the overflow bottle, it is full and sometimes overflowing. When the car is cold, the bottle is at the cold level as it should be. Also seems that if you loosen the radiator cap, a lot of air comes out of the system. I have bled it using the bleeders at the thermostat and water pump, have tried to get the air out by leaving the cap off and idling the car, adding coolant when the leve drops. When I do this though, the radiator overflows and then drops and then overflows and drops again, loosing a lot of coolant. There never seems to be just a constant flow through the system. What could be causing this?