Been having exactly the same problem - except once the boiler reaches temperature, it will not re-ignite. It also baffled my engineer, until he contacted a Baxi rep., who seemed to suggest that they were aware of the problem and had introduced a retro fit part (the boiler should go into 'pause' state when it reaches temperature, but due to the fault, it goes straight onto 'off'). The cost of the part is £260 + VAT, which I'm refusing to pay - I'll post an update if and when it's reolved.
Russ, November 2009