Alfa Romeo Spider won't run unless AFM disconnected

The intake and throttle body could be dirty, as well as the Idle Air Control Valve, (IACV), and the Fast Idle Thermo, (FIT). If the Intake tract and throttle body are dirty, that can affect performance. If they are dirty, the IACV, (which controls the idle as the engine load increases from; power steering, air conditioner, headlights, other accessories, etc. The FIT controls the idle in sort of the same way, but it works when the motor is cold like a choke and it can fail which can affect the idle even after the motor is warmed up.

There are also emissions stuff like Exhaust Gas Return (EGR), Positive Crankcase Valve, (PCV), etc. Which can have some affect but not as much as the other items.

The AFM itself might be the problem or maybe a combination of a few of the items mentioned.
It sounds like you maintain the car very well. So, I would sugest starting with cheaper items and go through things one at a time.

First get some BG44K, which is a fuel injector and combustion chamber cleaner. You will probably have to go to a Dealership to get it. Run 2 cans and then change the oil. It works so well that it gets the oil dirty from deposits that it removes.

The next thing I would do is to clean the throttle body and intake manifold. When you remove the throttle body, you will see 1 or 2 openings when you open the butterfly by pulling the throttle cable or turning cable pulley. They lead to the IACV and FIT. Be careful not to let stuff get in there as it will make them clog. You can remove the IACV and try to clean it, the FIT is sometimes cast into the throttle body. The IACV generally has a screen and an O-Ring. So, you could clean the screen and get a new o-ring if you remove it.

So, try to clean things first as the IACV and FIT can cost a lot of money.
Try the BG44K and see what effect it has before you get into the throttle body stuff as it might take care of the problem on its' own.

Let me know how it goes.

One other thing you might try is to get an infra-red temperature gauge. On the exhaust manifold near the head, point the gauge at each respective exhaust port. If one of the cylinders varies by more than 30 degrees Fahrenheit, then if that cylinder is not firing.
If you don't have a temperature gauge, drop some water on the same area and if it takes longer for one of the cylinders to boil of the water, then the same applies. Let me know what you find.

Try running if the BG44K. It is the only fuel injector cleaner that really works. If you can't find it, Sea-Foam is the only other fuel system cleaner I would use. It really can make a huge difference in performance.
Replace the air filter just to eliminate it as a possible suspect if it is dirty.
Run the injector cleaner and let me know the results.
The next thing to do if if the problem still persists is to look at the IACV and the FIT.
I will watch this post for your reply even if it takes a while, I will respond ASAP.