Central Heating Control?

Every ten days or so the central heating system ignores the programming set into the clpock programme. It remains on constantly and I suspect (from both the room and water temperature) that the boiler switches to maximum output. Reducing the setting on the wall thermostat has no effect. At the same time, some radiators will be very hot and others cold - this bares no relationship to those fitted with integral thermostats nor the individual room temperature. Turning off the electric supply to the system for a few hours appears to reset the system and it will return to normal controlled operation. The problem was discussed with a B.Gas engineer during the boiler's annual service and he had no ideas as to the fault. The boiler make is Baxi, mated to a Lifestyle LP241 programmer.