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Idling Problems of a 200TQ

I'm having a problem with my '89 200TQ.  Shortly after the car is started
(approx 5-10 secs) the idle becomes extremely unsteady.  It begins to bounce
between 1000 and 1700 RPM rather quickly.  I have checked for intake or
vacuum leaks and have found none.  Also when the OBD is checked for fault
codes, none appear.  When the throttle position / idle switch is unplugged
the engine speed increases immediately.  I have a spare ECU "brain" unit for
the car and there was no change when these were interchanged.  Also it seems
as though when the climate control is turned to AUTO and the A/C compressor
runs, the idle steadies to about 1100 RPM.  
Please note that the following part have been changed within the last year
and a half and therefore do not seem like possible problem areas: O2 Sensor,
cap, rotor, plug wires, plugs, temp sensor(on upper rad. hose gooseneck).
 Also the idle control valve has been cleaned to remove any carbon build-up.
This beast has 170K miles on it and would like to see many more, so any help
would be much appreciated.   THANKS!!!!!!