Baxi system 35/60 ignition problem

Hi all, I have the above boiler and it has developed a curious problem. When the demand is called, the boiler needs to be reset 4 times, the first time the fan runs, but no spark, ignition fail light comes on, reset it and fan runs, spark but no gas, ignition fail light comes on, reset it fan runs, spark comes on a puff of flame, ignition fail light comes on, reset it and on the 4th time it runs as it should, until it reaches temperature and then you have to go through the same sequence again, fan switch is ok and operating, I have checked flue, all is fine, temp switches seem fine, I'm thinking the board, but why does it start on the 4th go every time, thanks in advance