my '90 V12 with Marelli had that occasional idle stumble several times reported by other owners in this forum. After looking at the common culprits (CPS, white cable etc.) I finally found the solution for my car: The TPS connector on the harness side, usually buried somewhere down the firewall, showed cracked insulation immediately where the wires exit the molded connector. I separated all wire strains and made sure that there is ample clearance between the wires. Since then, the car is running very well without any more stumble. I suppose some bare wire strains occasionally connected to each other causing electric shorts to the Fuel injection ECU. I obviously need a more permanent fix as water on the bare wires can cause continued trouble.
Perhaps that helps some owners.