The engine temp on my wife's Holden Astra seems to be going all over the place. Start off cold in the morning and drive for 20 km. Heater works fine but temp gauge doesn't move off cold. However if I have to stop at a set of lights the temp gauge rises to about normal. When I start off again the gauge drops to zero. About 2 weeks ago I found the water level very low and topped it up. The motor does not use water but sometimes it smells a little like overheating but is not developing a problem as temp seems to be fine. Could this be a faulty thermostat? Or what else? Jim

I would think thermostat as my first guess as well.