Temperature gauge going down while driving

Check the coolant level but I think you need a new thermostat.

depends on the air your driving into. If the electric fan is not working which it likely is not it won't keep the car at the proper idle temp. Regular temp for an audi is betwen 90-100 anything over that is over heating. But as F_O_R suggested its possible the thermostat is acting up. If that is the case it would be a good time to do a timing belt, water pump, thermostat, pulleys and hardware change.