Given the ... marginal ... state of ground circuitry from Italy, it's entirely possible that you have a wire rubbed through on the framework, with a slight touch introducing voltage into a poor ground, neutralizing the ground path to your tach (and other stuff maybe)
You can do some testing by comparing voltages- across battery + to ground, battery - to ground, and ground-to-ground across different points. It's not a guarantee that you'll see a differential, but if you do it's a certainty that something needs to be addressed.