My car stopped producing heat a few months ago. The thermostat was replaced first, still no heat. The heater core was flushed, no heat. I also replaced the temp sensor, blower motor and water pump and finally the heater core. Still no heat! At idle the cabin gets warm, when increasing Rpm's the air becomes cold again. I pulled the dash apart and got to the blend doors. I moved them manually and they are not stuck or broken. Any solutions?