Baxi Bahama 100 burner cuts out

Hi, I have got a Baxi Bahama 100 with the honeywell valve and pcb. It is about 4 years old. When I turn on the hot water or central heating the main burner lights but cuts out in under 8 seconds.
However when turn on the water or central heating for a second time (sometimes it can be three of four times) it fires up normally and the boiler will be ok for a day or so?
I have replaced the flame sense electrode and checked electrode lead and the burner pressure is correct. The system is vented properly and the vent on the boiler is working ok. Would appreciate any help on this one. Thanks David.

Pump proving switch is my bet..the connections become loose and give this intermittant fault.

Got to boiler on Monday checked connections - your bet was right. thanks David

davidb100 said:

When I turn on the hot water or central heating the main burner lights but cuts out in under 8 seconds.

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Primary flow switch (Is/Will soon be broke)