Thats a nice looking little car. I am sure you can make it even better with your additions
Map sensor connected the right way? Gasket seated properly on TB and secure? Throttle cable not slightly too tight/catching?
Simple option would be refitting your old ECU and seeing if it still does it. My money is on yes.
https://www.fiatforum.com/cinquecento-seicento/258767-throttle-body.html
post no.3 and label 10 is MAP takeoff. From what ive read you bought an ECU with McKrich chip fitted thus bypassing Immob, so fitting the old ecu breifly will rule out that side of it. Try unhooking the throttle cable and seeing how it reacts. You can also unplug the ICV to see what happens. Doubt TPS as it would prob drive like absolute crap.
How big are the holes ?