Potterton Performa 24 combi boiler on off cycling problem?

My Potterton Performa 24 combi boiler ignites correctly but only stays on for a minute initially then switches off then re-ignites only for seconds then goes off again for long periods. This is the same for DHW and heating - no pipes get hot enough to cause a temperature sensor to switch. After following fault finding guide the PCB was indicated as faulty - after changing the PCB the fault is exactly the same - Any Ideas please