I think follow the advice above and check that the sensor is properly in place and keep an eye on it. I think the coolant temp sensor fault would mean this happens frequently. Have you just changed the coolant?
When driving Trophy first thing in the morning it take a while to get upto temperature, when its in the middle of the gauge is then drops to 1/3 way up the gauge, then goes up to halfway again. Is this a faulty temperature sensor? seems ok at the moment, just thinking that in cold weather is could cause an issue?!