Vokera 12_24 RSE Mynute Central Heating Boiler Burner keeps going out?

I have a Vokera 12-24 RSE Mynute Heating Boiler(Direct System)When the room thermostat calls for heat the pump starts first then after a couple of seconds I can here the water flow switch activate followed by the fan kicking in. Straight away the differential switch clicks and the boiler lights.

Once the system heats up then the burner keeps going out and relighting again, the time varies from 3 - 15 seconds, each time just before the burner goes out I can here the pressure switch click and again just before it re lights.

I have replaced the pressure switch and the small hose to the fan is clear. I have also removed the flow switch and checked the spindle is free. I think the fan could be at fault maybe not keeping enough pressure on the pressure switch. I have printed off the manual and it points to cycling and the fan or fan connections.

Any advice would be most greatful.