linea 28 hot water problems?

my linea 28 hot water problems are that when hot tap is turned on water is heated to 40c then burner steps back and temp levels out at 40c which is no good having look warm water. i have changed thermisters x 2 ,serviced the diverter valve new heat exchanger i am at wits end one last thing when a dry thermister is put onto dhw side boiler runs full up and get lots off hot water