2001 VW Passat. Engine temperature gague not working
 

On my 2001 Passat TDI 130BHP the temperature gague has stopped working all of the time.

Most of the time it sits on 50degrees c. On random occasions it starts to work again and will reach normal temp of 90 degrees c.

Any idea what it is?
Possibly a faulty sensor? Where is the sensor located?

and if by any chance someone has a instruction guide on how to replace the sensor that would be great.