Hi Temp gauge acting up. When start up car warms up nicely to 90c and stays there for anything from a few minutes to half an hour or more, then steadly drops to 60 stays there for a varying length of time several minutes then rises back up again may hand around 75 for a while before going back to 90, then repeats process. Car runs fine and pulls strongly and has been doing this for 500 miles or more. This happens on short and long trips. Is is probably sensor and is it an easy job? The car has done 79800 miles so due for cambelt change soon.