I have a Baxi solo 3 60PF on a conventional gravity fed system. I have a problem where the boiler didn't ignite at all. I have had a corgi registered guy out and he has replaced the pcb, fan (said it was heavily worn). Now been getting a problem where the boiler run for about 5 mins then starts to cut out every 5 seconds. So he replaced the microswitch for the fan. He now thinks that there is a problem with the flue. With the exhaust fumes getting back into the boiler, because it stayed alight when the front cover was opened slightly, I have now found that even if you do this and run it for another 10 mins it still goes out and re-lights every few seconds. Any ideas as to what the fault is?