I had this problem some years ago, I could not set the oil pressure at the burner by hit and miss, it either would not fire or would not stay firing, in the end I fitted a pressure gauge in the supply line and set it to 80 psi as far as I remember, it was fine after that. Setting the correct pressure is critical, I think there is a connection on the pump block for a pressure gauge but I fitted it in the line via a tee piece.
On another topic: should there be vapour at the flue outlet to atmosphere? it is pouring out, the ground under the outlet is always damp. Thers is a condensate drain pipe almost at ground level, a little water comes out of this pipe. The Plumber who installed it said this was because it was new, that was six weeks ago. I understand a "Condensing Boiler" has the central heating return line passing back through the flue, picking up some heat which would otherwise be going to atmosphere, therefore making it more efficent but Iv'e never seen this before and I am sure there are hundreds of them around.