I have a recurrent problem with the immobiliser, the immobiliser, and dare I say it again?, the immobiliser on my Alfa 156 2.4JTD.
The fault first occurred 4 months following purchase from new, and 18 months on, the local dealer has so far changed the electronic management unit, the ignition keys and repeatedly tested and corrected faults in the wiring loom (connections and termination points) from end-to-end.
I have not been away from the garage for longer than 3 months at a time, and still, the same fault persists.
When the ignition key is inserted, the immobiliser light stays 'on', which does not allow the car to start. You have to repeatedly remove and re-insert the key, offer up a quick prayer, then try to re-start if the immobiliser light finally goes out. It's completely hit-or-miss whether it starts or not.
Being an intermittent problem makes it even harder to actually demonstrate to the mechanic. Every time the problem is once again 'fixed' at the garage, it's now a question of how many days/weeks/months may pass before it happens again, which so far, it sadly, but inevitably does.
The other (newer) fault appears when driving and the accelerator pedal is depressed, eg when overtaking on a motorway. There is, for an instant, no power whatsoever, resulting in a momentary 'lag' before the power is delivered. Pretty scarey when you are overtaking at speed.