car has heat then gets cold then warm again new thermostat fuilds full

make sure your heater core isn't clogged if it gets warmer as you step on gas more than likely it is clogged

Clogged heater core as previously answered or possible air still in the system.

It could be caused by couple of items. There could be low flow in the cooling system, plugged heater core or still air trapped somewhere in system. If you would like us to look at it just call 313-562-7022 or schedule an appointment online at StansGarage.net. I am sure we can figure it out for you. Larry

sounds like low coolant level