Ur cars Idle up system is not working. This system maintain the Idle RPM by slight increse in throtle position wen Elctrical load Like (Defoger, Radiator Fan, Heater Fan, or Head lights etc) are turned ON. Its mechaniism is almost same as the system which increases ur cars engine speed wen u turn On the air conditioner. Either the vacum pipe connected to this system are disconnected or u just need to play with adjustment screw.
In worst scenario the Vacum Switching valve or Vacum Actuator cud be the culprit.