does anyone know hy my vokera linea 24 cycles on and off?

my boiler lights the goes out every 5 or 6 seconds on heating and hot water flow and return clear pump running aps working fan working motorised head working have cleaned out plate to plate heat ex checked electrode connections at first i thought there was some dirt in heat ex or boiler parts causing it to overheat or stick somewhere but after 2 hours of investigation and checking micro switches and venturi it leaves me , with ignition box fault , pcb or something else .if any one can help it would be apprieciated thanx