Here is some advice from Alfa Bulletin Board, where more tips and fixes are available.
The GM sourced engines have issues with timing chains stretching and the diesels have to have an Italian tune regularly or the EGR system in them clogs up requiring the removal of the head.
I’ve had a problem with second gear slipping to fourth gear then slipping into neutral. It has been an ongoing problem that my mechanics have yet to fix. I’m hoping to find some help online so when I take it back to the mechanics I can give them a list of possible causes that they haven’t investigated yet. Someone suggested Alfas are known for actuator problems? Can anyone provide any help please?
As far as I know the 159s have cable connections between gear lever and gearbox selector. So first check is that the cables are correctly aligned.
Second check is the gear selector in the box, I don’t know if it’s a box out job.