Temp Gauge reading

[Spaceman]

Hi, I'm having a problem with the temperature gauge on my A3 1.8T Sport ('98). When I drive the car the gauge only reaches the 70 mark. The gauge only ever reaches 90 when the car sits idling but it will drop back to 70 once the car is moving again. The blue temp sensor has been replaced but this hasn't resolved the problem. Is there another temp sensor? Any ideas as to what the problem could be?
Thanks

:boggled: