Potterton 50L?

Boiler sequences through to main burner ignition but only for 5-7 secs.Boiler then recycles with similar results.
It has the new PCB Module fittrd(ie 1 LIGHT: Grreen/ Orange)
Burner flames pulsate and create some vibration( Lack of Air ???)
REmoval of Front Cover remedies problem.

Is the Fan on its way out ???
Its not blocked and it seems to be turning but thats when it does nor need to suck the air through the pipework and boiler casing.