Temperature gauge not functioning

if the engine is getting up to temp ok and you say that when you are sitting in traffic the guage moves up and drops when you start moving this is normal as the water is flowing through the engine and radiator because air is flowing through the veins of the radiator and the water is then passed through the engine hence the temp dropping so i would say there is nothing wrong with the car but if you are unsure then replace the multifunction coolant temp sensor available from all good motorist discount centres