Recently, when driving on a motorway after 20 minutes or so, the temp gauge of my 2005 Mondeo 2.0-litre petrol automatic moved into the red. However, if I coasted downhill, the gauge came off red. The car seems to run ok and not boil over, although there is evidence of wet under the overflow tank. The AA pressure tested the system and found no leak. I am now scared to drive any distance in case the car breaks down. Will a garage know what to check without just replacing all the coolant parts? Any suggestions where to start looking?