If the distributor has a vacuum advance I'd suspect that. On light throttle or on the over-run the vaccuum advance mechanism should advance the ignition (this is when the vaccuum in the
Fulvia Distributors do not have vacuum advance.
After checking the timing you might have some issue with blocked idle jet especially if the car has been standing for a long period of years before being used. The pilot jets are accessible under the small cover on top of each carburettor. They are the shorter of the jets (2 per carb).