Auto Parts: Alero 1991 heater blowing cold air, heater core, alero

Question
My heater blow cold air at idle. It only blows warm air when accelerating, but when i am slowing down it goes cold again. Any ideas?

Answer
I have a feeling that your heater core may be clogged from your description.  My theory behind this is - at idle, your water pump isn't spinning very fast so it's not enough to push the coolant through the clogged heater core except when the motor is spinning faster during driving.