What temp did it used to get to ? I have no experience of this car, but the vast majority of fiat's, and tbh most other car manufacturers, the water temp dial heats up to halfway (or just under/over)
As long as the temp doesnt rise above 1/2, or near enough 1/2 i would imagine the fan is working.
Sounds like you have the same issue as my punto. A sticking thermostat.
Whats happening is your thermostat is more than likely stuck up open, so when your driving and accelerating, due the the increased rev's the engine is pumping more water through the radiator, but the thermostat isnt closing to compensate for it increased airflow through the radiator, hense why the water temp drops to 1/4.
I have had 3 thermostats from car parts shop now and all have done the same thing, so for my next i will be getting a genuine fiat one.