Take it you know that the engine isnt actually running hot? Checked all water levels and belt tension of water pump etc??
If it's none of thoose i agree with 'Blackburn', get your multimeter and check the voltage going to the gauge.
If you can check that the needle goes from the white too the red so that you know the gauge isn't faulty. Do this by applying a voltage of around 10-12V on the gauge.
If all is well and good and it reads to both extents it could be the sender unit but highly unlikely as it is new.