when you start the car do you wait for the code light and injector light to extinguish before fully turning the key and starting the engine as this has caught me out before, although i never had the code light flashing?
do you have the owners manual for you car? it should have something in there about it. You can also try and put the immobiliser code in manually see if that clears it up all is explained in the code part of the manual which i will look at later and see if it shed's any more light on it?
Hae you tried disconnecting the battery (negative terminal) and reconnecting again after about 15 mins or so?
Recently, my CODE light wouldn't extinguish when I turned on ignition and hence wouldn't start. Thought it might be problem with ignition key but had same result with master key. Disconnected and reconnected battery as above and its been fine since. (just one of those 'Alfa quirks' I s'pose - though I do now carry a 10mm spanner with me just in case it happens again!)
I have just had a quick read of the manual and a flashing code light suggests that the car is not protected by the immobilizer and that you need to get the keys codes re-stored into the cars memory. Have you tried to use your master key or service key and see if it still happens?