First thing I'd do is clean the throttle body and be sure the throttle position switch (TPS) is doing its part. If you're not familiar with this, check out the 7/9 FAQ list from the drop-down selection above. Those cars work the same for this function.
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Art Benstein near Baltimore

Reassembly is the reverse of disassembly. Except the profanity. You don't take that back; you just keep adding to it.

Hi,

If you pull up to a stop light and it occasionally dies, then it could be a crack in the accordion tube or vacuum leak that gets shuffled with engine movement. You are not describing that!

It sounds like the IAC valve is not responding to a load changes that effects the RPM’s. An automatic transmission is more subtle than a manual shifter.
It’s probably dirty or approaching end of life. Getting sticky in either case!

Life varies quite a bit due to how many times it’s required to correct idle speed.
I have seen them wear out around as little as eighty thousand miles to well over one hundred and twenty thousand plus miles.
They are a carbon brush motor and will eventually wear out. Honestly, Not very predictable?

You have to listen to the engine RPMS for it to race the engine up, above its normal idle, every time it’s STARTED. Cold or hot operation!
It is then supposed to immediately bring it back down to a normal idle of 750 rpm that is required by the ECU computer.
This does not trip any CEL codes that I know of because there is no feed back for such a thing.
It something you have to consciously think of it, because we get so use to it working flawlessly.

IACs have been discussed on the BB in that prices vary greatly and the quality might as well?
It is under a watchful eyes as either one maybe be coming from China or countries other than in Bosch Germany anymore. The system was used by many car makers so the tooling moved around the world.
In order to clean or inspect it, it will have to removed.
Having a spare makes it a more productive endeavor!

On the later cars, above 1986 or ‘88 they are spring loaded on the return stroke and so it’s a two pin model you are looking for.
On both three pin and two pin use a small nine volt battery to test them under starting or low voltage moments for internal brush sticking. A three pin gets it voltage polarity reversed so I alsoI test them with the quickest wrists twisting I can muster up.
They both wear a tapered groove into the commutator from going back and forth a halfway turn.
Good ones snap open and close smoothly but have to repetitive electrically!
This is what prompted the spring return on the later cars.

Hope this all helps fit into your symptoms that you are experiencing!

Phil

More info

Is anything on besides the motor? AC, Lights, etc

Do you know what the actual idle speed is? Using something like a tachometer in a test kit.

Fuel, spark, air

Have you looked at codes in the test boxes?

Have you had the car tuned recently?

How does your air cleaner look? I had a '90 744 TI that would not run well because the foam lining of the top of the air box had turned to powder and clogged the air filter.

Do you have AAA?

Do not throw parts at it, I would have the car towed to a dealer - most will only charge one hour Maximum for trouble shooting. My son-in-law is a Subaru tech and that is Subarus policy also.

We have owned 9 Volvos and driven almost 700K miles - I have our cars serviced by a dealer and never had them replace more than one part to repair our cars.