Picked my 1.8T A6 up today. It was fine for about 10-15 miles, and then the temperature went up to about 110 degrees. If I dropped my speed to about 40, it would then drop back down again. At slow speeds (road, not necessarily engine) it seems fine, but go faster and it creeps up. Any ideas on what it could be? Temp sensor? Thermostat? Water pump?