Sounds like the throttle valves are not closing fully, or the idle speed motor is passing too much air, or there's another intake tract point of entry. If you can push the throttle valves closed to the point where the idle returns to 900 rpm then its probably an issue with the valves or linkage binding due to grunge or wear. Once fully closed the idle motor should step in and regulate airflow to the intake side of the equation until a steady 750 rpm is attained. If it never drops to 750, then its because there is too much air entering the intake tract. Your job should you choose to accept it is to figure out where it's getting in :-)