I agree--are your heater controls working smoothly?? It's probably the flapper door in your heater box stuck if all mentioned above checks out. The heater controls and the box would be the first place i would look also. PS-- Probably be the easiest thing to start with.Could also be that the heater door isn't adjusted correctly; the control wire outer shield could have come loose. Look under the dash and watch the door control when you move it up and down.