Myson Economist switch off problem

I have a Myson Economist boiler which is connected to an electronic timer. The problem I have is that usually but not always the boiler ignites and the radiators are heated when the timer comes to "on".
However more often than not, if the timer is set to "all day" the boiler will switch off and all heat goes from the radiators despite the timer saying "on all day". The bolier seems to be making a whirring noise but is not ignited.
This can be fixed if you press override on the timer, switch hot water and heat off and then switch back on again. The boiler ignites immediately.
I've had loads of engineers out and they all say they can only diagnose the problem if they are standing by the boiler when it happens! Unless they camp out I can't see how I can sort this.
Can anyone point me in the right direction for what might be wrong - I'd be so grateful because I am getting very frsutrated with the "experts".
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