Well, I checked both throttle position sensor and idle fuel cutoff switches. Both are working good and consistently. Then, I remembered reading about a stiff linkage, so liberally sprayed lube on it. Now, idle huting is gone. Probably throttle linkage was not allowing for throttle to return all the way and TPS was not sending a "fully closed" signal.

New pro0blem is that it now idles consistently high at around 1,200 to 1,500 rpm, after a warm-up period. It fast-idles at around 1,800 and goes down to 1,000 or so. Once fully warmed up, it comes back to 1,200 to 1,500 range and stays there. With little fluctuation but not something I call hunting (rapid successions of ups and downs).

As I mentioned, engine runs perfectly at smooth 800rpm when idle air control valve is unplugged so I don't think it is a vacuum leak.

Any thought?