It happened twice, always on the same road. I was on a mountain town, and I had to hill up a bit more, before going down in the valley. During the first part the engine reached the usual working temperature, but in the middle of the descending part I realized the temperature was completely at 0 (and it was correctly indicated, since even the heating air was cold). After the first time, I added a bit of cooling fluid, but it needed just a little bit. A couple of weeks later, on the same path, the same thing happened. Someone suggested me to purge the cooling circuit from possible air bubbles (I will post here the procedure, as soon as I do it). Anyone else experiencing something similar?