Much as I love Alfas, these Selespeed boxes are a nightmare.
Sounds like either low pressure in the oil pump or an electronic issue, which isn't uncommon. Your best bet is taking it to an Alfa specialist, a normal garage won't have the know-how to deal with it.
To be honest, it may well be the end of the car because they ain't cheap to fix and the chances of getting a working secondhand box are slim.