I think the answer is Andreotti although it's a strange way of putting the question: he was PM from 1972-74, from 1976-8 and again in 1989-92 - so he was only PM for seven of those 20 years.
yesh God I was gonna say....The italians changed their governments more times than....There's one space when they clocked up over a hundred govts in a ten year space.
It was something to do with proportional rep -and never getting a majoirty