Could be one of a number of things. Try pumping the top hose with your hand whilst filling the rad, see if you can shift an airlock in the heat exchanger. Otherwise more info req!
Oi, nothing wrong with little Fiats: when I was 17 I got my mum's Uno 1.0 i.e. up to (and a little over) 100mph down a slight hill on the M3 with a tail wind. Although when I braked the heat made the discs deform and vibrate the car...
A Fiat Stilo is the only car I have ever driven with traction control that does exactly what it says on the (spaghetti) tin so they can't be all that bad I wouldn't buy one, mind.