I'd be leaning more toward a false air leak myself, particularly as the system goes into open loop (off the O2 sensor) when idling and uses vac to help the idle airspeed device maintain steady tickover.
I had the same problem on my '91 (it was up and down by few hunderd rpms at a time) which was altimately cured by replacing the idle spped stabilizer. In most case however, you can take the stabilizer off of the intake plenum and clean inside of it with a carb cleaner to fix the problem. The stabilizer is located by the throttle position sensor, a shiny cylindrical (about 1.75" in diameter IIRC) object. It carries a Bosch sticker w/ 0 280 140 516 (part #) printed on it.
First make sure there are no un-metered air leaks.
Remove the intake hoses to allow access to the throttle plate.
The throttle stop screw becomes worn and the plate will become (slightly) clogged with carbon/oil.
These combined will cause the restriction of air flow.
Clean the throttle body/plate and reset the throttle stop.
You may have the remove the small plastic cover to access the stop screw.
Set the stop screw so you have .001 to .002 (inches) of clearance at the plate/throttle body.
The idea is that there should be some air flow through the throttle body when closed.
If there is little or no flow, the idle air motor works to extreme measures to control the idle speed.
This causes swings in idle speed. It will eventually find the right speed and idle at 1000 rpm. But, will surge up and down for a while until it settles.
By cleaning and resetting the plate there will be some flow and the idle motor doesn’t have to work as hard. It will only have to make small adjustments to the air flow to provide the correct idle speed.
Depending on how much the stop screw is changed, you may have to reset the throttle switch. Loosen the two 5mm bolts that mount the switch and adjust it so you can hear a “click” when the throttle is opened.
I am having same problem when hot (91 Spider so Motronic so no AAV). When I unplug the electrical connection from the idle speed stabilizer the idle momentarily slows than evens out, so it idles better with the "stabilizer" unplugged than plugged in. I have not yet tried the suggestion above to clean it. I do not hink I have any false air leaks. And what is the "donut" referred to for the stabilizer in other threads? TIA!