I have a similar problem, have had the circuit board replaced twice, the controller replaced. In fact the boiler has been largely rebuilt over the past 10 years but still it has a mind of its own. I dosnt seen to develop pressure it clicks on and off and the pilot will not light - then for no reason... bingo! on it comes.... no one has been able to provide me with a solution - I am considering changing the whole system! Sorry I cant add a solution.
Richard
March 2005