Re: Cooling Problem with 3.0 V6
It seems highly unlikely that the impellor would slip when rotating slowly & then work when under higher load - unless the belt is loose & not turning the pump properly (don't know if it is the aux belt or the cam belt without checking).
I have never known the fan to continue when engine off (does on wifey's punto, but not on my 166).
Temp gauge should only go high when there is a real problem - the dash gauge has some sort of anti-scare device & reads 90 all the time when running unless major variation.
Rad & thermostat check & full flush might help to diagnose the fault.
Simple check for the thermostat - start engine from cold, hold top rad pipe & it should get hot suddenly, rather than gradually - this is a sign the thermostat has just opened.
Waiting for an Alfa version of a Maserati Quattroporte