I have a 1999 Escort that has an erratic idle whe the heater system is on anything other than floor or vent. If the floor and vents position is on or any combination with the defroster the engine will idle fine then lug down and stumble and almost die, then idle fine and lug down and stumble and almost die, continuously. Above idle it also does this only it's not noticed as much. Also noticed it doesn't seem like as much air is coming out of the defroster. The problem seems dependent on switch position. It is 10 degrees and I wouldn't think the air conditioner would be kicking on. When it's on floor and vent it shouldn't kick on there for sure.