I used to work for Fiat and the most common fault for this symtom is the oxygen sensor (lambda).
You need diagnostic equipment to test it but it can be done with a decent multimeter and reasonable knowledge.
Basically the sensor once warmed up provides a signal to the ecu to switch the fuel mixture from lean to rich, this switch should occur approx 2 times a second. It does this by sending a voltage signal to the ECU. Rich = 0.1 mV and lean = 1V. There are othe common complaints so before swapping parts unecessesarily, the oxygen sensor is 60 pound from Fiat it would be worth a diagnosis test. Godd luck