The answer is that You don’t need a Partita IVA if your job is not running regularly.
In this case, your job is regulated as “Temporary job” (lavoro occasionale).
You may have heard that You can work on a Temporary Job only until € 5.000 per year.
This is not true, because the 5.000€ ceiling has been canceled a few years ago, so now the only criteria to work without P.IVA is only “qualitative”.
You must remember, however, that when You exceed 4.800€ you must inform your Employer, and your employer will start to pay your social security contributions on your behalf.