My 130,000 mile B7 Audi 3.0 Quattro Avant has had a fluctuating water temperature gauge, moving from 90C and upwards eventually bringing up coolant level warnings and engine going into limp home mode. Nothing wrong with coolant level, with a healthy colour to it and no evidence of overheating. Car went to very helpful local garage, Hennell Vehicle Services of Henley-on-Thames. They thoroughly flushed the coolant system and on my instruction, replaced the water thermostat and water pump ( I have read on this website that water pumps can fail around this kind of mileage). Problem has now completely disappeared.