worcester heatslave sparking running on?

Hi my Worcester Heatslave 20/25 burner cuts in & out. The motor runs OK, the start sequence is OK in that motor runs, sparks are generated & the oil ignites, the sparks (ignition) stops after a short while. After a short time after 5 - 10 seconds the burner cuts out (fan is still running), the sparks start again, a short time after I presume the oil pump opens & the burner starts again. The sequence then repeats over & over. I have replaced the sparks ignitor as it was taken out presumably from being over worked, photo cell & oil pump all changed - Any advice ?