What's wrong with the Flexiflame 140?

The burners go to low flame (about 6mm) when the room stat stops calling for heat or when the programme ends i.e. at night. When the boiler pump (integral) stops, as it does when not required, and the burners stay at low flame: the heat exchanger starts to "kettle" boil and the boiler goes to lock out. Have vented the pump and purged the heat exchanger