We recently had a lockout on the boiler and CORGI guy fitted new PCB. (£200) Boiler worked again but was turning on and off too much. He recommended a powerflush as he said the circulation could be poor causing the lockouts. This we had done at great expense. Rads now heat up quickly but still getting occasional lockout. Boiler will reset ok and run for a day or two and lockout again.... I think it's a boiler fault rather than circulation. Could it be gas pressure or thermostat?