Im having a simular problem.
Currently running without a thermostat,and have been for two weeks, temp gauge always sits on the bottom of 'norm'. Fans still cut in and out fine.
Driving home last night, after 25miles of 30/40mph off boost, the gauge drops off the bottom for 2minutes, the rockets to the top in under 10seconds.

So i pull over, let the car cool, restart and its halfway on the gauge, make a bid for home and it drops off the bottom again, just as i pull onto my driveway, it shoots into the red.

The fans cut in and out fine still, but seem to make no difference to the gauge. Dodgy sensor? maybe?