Idle at operating temperature inconsistent - idles between 500 and 700 rpm

Hi, I've got an idling problem at operating temperature. The vehicle is a 1998 Toyota Corolla 1.6 automatic UK specification (OBD1).

Complaint: Engine sometimes idles low at a junction, drops from 700 to 600 rpm, but not on every occasion. Sometimes the idle goes down to around 670 rpm but picks up again to 700 rpm at junctions, other times the idle is stable. Putting the transmission into 'R', the idle goes down to 500 rpm but picks up again to 700 rpm when the transmission is put into Park. The idle is not rolling between 500 and 700 rpm.
There is an rpm flare up upon hot start up and there is a cold fast rpm idle which is not doing anything unusual. The car drives perfectly fine with no other running issues.

Tests I have done: No codes, TPS is 0.56 volts with throttle closed, MAP is 1.5 volts engine idling, O2 sensor is oscillating normally, Vacuum gauge reads 20 at idle.

Idle air control valve testing:
Duty cycle cold (engine first started) = 77%
Duty cycle at operating temperature = 45%
Duty cycle at operating temperature with AC etc on = 54% - RPM rises to 850
Duty cycle with transmission in 'R' = 47%
Duty cycle with steering wheel turned = 49%

Duty cycle percentage readings are when the computer is grounding

Work I've done before taking the duty cycle readings: Cleaned the idle air control valve, which was sticking badly but is moving freely now. O rings are present and the vacuum hose is good. I do wonder if the throttle body is still dirty.


Any ideas what could be causing the problem please?


Thanks

Stephen

Yes I would clean the throttle body it might not be that but it fits the symptoms and is an easy job.
While I don't think it is a vacuum leak however a flare up in revs at cold start is sometimes a symptom so it is worth checking using Paul's water squirt method.