Water temperature problem

Car was runing well with temp steady. suddenly temp dropped and became erratic. I have changed the thermostat ( twice ) the temp sender, The water pump and timing belt and have flushed the radiator,found to be clean. The problem is still there, ie lack of temp control it goes up to 110 deg and slow to warm up,it also varies with driveing conditions ( the heater works according to the temp ) All parts are Citroen parts. The radiator does not apear to be very hot , dispite this the cooling fan now runs frequently as the temp guage reads hotter than it did after a run. ie it origionally controlled at dead on 90 deg.