My fuel gauge does the same thing. I was hoping it would go away as I drove it more. Might pull it out and clean the contacts this week since the tank is out for a filler neck seal replacement.
LOL, Fiat is horrible about electrical things not working if you don't drive it enough.
Maybe even the older cars are starting to take on the Fiat mantle. :-)
Probably grounding issue with both of you're cars.