Intermittent fault with Mondeo '01 heater and temperature gague,?

Ford Mondeo 2001 a connected problem, the temperature guage works intermittently and when the needle is resting on cold so is the air coming out of my heater but when I may go over a pot hole or round a sharp bend it will come to life and start to rise to the middle of the gauge, it may get to the middle it may not and if it does it doesn't stay for long, why does it do this and how can I fix the problem

Neil
January 2010