I've always noticed that the Idle Air Valve is very sensitive to accumulated deposits - that fine black slightly oily sooty stuff that gradually accumulates in the throttle body and on the butterfly flap.
Have you tried taking off the convoluted air duct, holding the throttle open with the wheel, and wiping as far as you can reach inside the TB with a rag wound round a stick, soaked in carb cleaner?
Then remove the IAV and squirt in some cleaner. Clean the sliding valve with a soft brush, using a stick through the exit port to rotate the actuator. Check that it snaps closed. In fact it should close just a fraction open.
I've found this sooting up happens quite quickly - I make it a sevice item about once every alternate oil change. A symptom that it needs doing is if the idle sticks too high as you describe, and can be corrected with a blip on the throttle.
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