Why does my boiler cut out after about 5 minutes?

My boiler (Potterton Kingfisher Mf) cuts out about 5 minutes after it comes on or is turned up in the morning. After another few minutes I can reset it and then it's usually ok for the rest of the day. We had thermostatic radiator valves fitted recently and the trouble has started since then, though we have since taken them off temporarily and still had the problem. The thermostats in the boiler have been replaced and we are told the next step is to replace the pcb, which will be expensive. Any other ideas would be welcome. Thank you.