RX4 2.0 not overheating but..

..in heavy traffic the temp gauge has started to rise to 3/4 before the fan cuts in reducing it to just over midway. We've owned the car from new and the gauge has rarely moved above midway even in the heaviest traffic. The fan would appear to be cutting in at a higher temp than before. With the a/c on the gauge remains at the midway. I recently changed the coolant sensor when the temp gauge started behaving erratically after which it behaved normally until last week. The local garage checked it over and said everything's working OK. Is there any component that would make the fan cut in at a higher temp, e.g. radiator fan switch/faulty new coolant sensor/relay? It's disconcerting that after 14 years the temperature behaviour should suddenly change like it has.