Audi Repair: faulty temp gauge or am i mad, temp gauge, white smoke

Question
i have a 1998 1.6 a3 and when driving the car on the motorway the temp gauge goes down to zero but when driving around town and standing idle at traffic lights it goes to normal 90 degrees (i,e)the faster i go the gauge drops and the slower it rises plus it uses a lot of water . please help as its been in several garages and dont seem to come up with an answer :(

Answer
If the car is using water, then you have to find where the water is going before worrying about a gauge malfunctioning.  Is there a puddle under the car when you park?  Any steam come out from under the hood?  White smoke or steam coming out of the exhaust?  Let me know,  Jan