been doin that to me too and blew a rad hose on the motorway the other day. best bet is to try the thermostat first as its cheap and easy, might not be lettin all the water reach the temp sensor properly, or just as easily the fan temp sensor, should be on one of the top hoses and unsrew to change. only cheap bits to change but worth tryin. sayin that my zetec has always tended to run pretty hot.