How to fix my Potterton PrimaF intermittent problem?!?

I have a Potterton Prima F.
For the past 3-4 weeks when the boiler has been timed to come on, it fires up normally for both water and central heating. However on some occasions the flame goes out after the boiler has been on whether for 5mins, 1hr or longer, but for no reason, as we know the water is not hot enough and the central heating is also still not hot. The 3-way valve for CH/water is not jammed, the water pump is working but we do not understand why the boiler flame goes out. The plumber is also puzzled. Can anyone help?