hi lee, this is a discontinued boiler so i assume fairly old. the domestic hot cycling as it is, is due to the flow through the heat exchanger being insufficient and making boiler overheat and cut out on overheat thermostat. It then cools down and relights. This could be due to limescale or a dirty heat exchanger. If it works ok on heating its probably just the domestic hot water heat exchanger. or the diverter valve. Either way it needs cleaning or replacing.
johnokent, June 2011
