why does my main burner keep constantly lighting and going out?

Hi I have a baxi solo wm 40/pf bolier which keeps constantly igniting then going out. The pilot is lit and the fan is running, I have had a new pressure switch fitted which you can hear operating when the fan starts and stops.
I have also had the pcb changed, a new ht lead and a new electrode, but for some reason the main burner will light up then a few second later go out (you can hear the solenoid pulling in and out)this happens constantly until finally it just stays off. At this time the pilot is still lit and the fan is still running but the main burner stays off, until I turn off the boiler and restart it which just starts the whole cycle again. Any help would be greatly appreciated as I am nearly at the point of taking a hammer to the damned thing.
Thanks Brett