Uh oh, is my 550 immobilizer failing?

Perhaps a few possibilities.

Failing starter solenoid? You can buy a remote starter switch to bypass the immobiliser and start relays. This hooks up directly to the starter solenoid terminals.

http://www.mactools.co.uk/rate-mac-tools-products/heavy-duty-starter-switch-tester-with-test-light.

No key input is required. Make sure your car is out of gear before using it.
Hook up the remote starter between
1) the large positive terminal of the starter motor, and
2) the small terminal of the solenoid.

You can buy starter solenoid repair/refurbish kits for most starter motors. YouTube will tell you how to fit them.

Your starter relay could be playing up?

Your car is not right hand drive (I assume), but I suspect the relay is in a similar location (driver's footwell doorside panel). You could put a jumper wire on the relay.

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Thanks to MattH for his photo (RHD car).
When jumpering the relay, your normal ignition key "Start position" will control the starter. Jumper pins (sockets) 30 and 87 on the relay base.

Both methods bypass the immobiliser.

 

By the way, how old is your battery?

When you attempt to crank the car, can you get someone to listen for the starter solenoid attempting to engage. The battery may simply not be strong enough to crank the starter motor.

 

I should have said that both methods bypass the immobiliser where the starter is concerned. The immobiliser still has to enable the Motronics ECUs to be able to run the engine.

Experts: Does the 550 have a Nippon Denso starter? If so, which model.

 

To me it sounds like it could be as simple as the ignition key switch that is starting to be whimsical... might not be very expencive to replace. But to manually apply + to that wire when the problem occure would be quite easy to do when the cover under the steering wheel is off.

 

It looks like the starter relay is under the dash on a LHD car, judging by the orientation.

https://www.eurospares.co.uk/parts/ferrari/550-maranello/electrical-ignition/electrical-boards-19911

Item #13?

If you can’t “hotwire” the ignition as per Eric’s suggestion, look for power at the starter relay when turning the key to start.

I recently posted a wiring diagram of the alarm system which shows the wiring from the ignition key to the starter relay. Taz posted a pdf file for the same diagram.