1.4 xsara overheating?

my y reg 1.4 citroen xsara has started to show the stop sign on the dash as soon as the car gets hot the temp gauge goes well above the 90 and this happens? the head gasget has not gone the thermostat has been changed the system has been bled and there is plenty of coolent in the tank also the rad is new? the fan comes on but very late is there a problem with the ecu and would only the main dealer be able to pick the fault up???