Boiler takes multiple times to stay fired up

[Ponx%4]

Hi there. I have Grant oil fired boiler which when cold, takes multiple times to fire up. It will run for say 20 or 30 seconds with hot exhaust gases then it stops. The fan is still going then it starts off again and stops again. After 4 or 5 goes it runs fine. When the boiler gets up to temperature it stops and is fine on the restart when the boiler stat kicks back in again it runs fine. Have had the oil pump replaced, the coil replaced and the photocell replaced. The filter at the oil tank is fine. Any suggestions please ??

 

[John.g]

Was the oil pressure checked before replacing the oil pump?.
Was the burner nozzle replaced?. were flue gas readings taken as soon as it starts firing continuously and again when hot?