Ideal Classic FF360 - Keeps cutting out?

Our Ideal Boiler will start up normally and burns for a period of time, but it cuts off the burner element after a while. The internal fan can still be heard running, but the boiler will not fire up again until after 20 minutes. It then burns for around 6/8 minutes and then cuts itself off again. And hence the cycle begins. We have fitted a new Printed Circuit Board and this has not fixed the issue. Any previous experience - help suggested.