Baxi 100 HE keeps tripping on start up?

Have been having problems with my boiler keep tripping on start up. Having spoken to the Baxi manufacturers the boiler engineer fitted a new PCB board. Still not working and engineer has been back on many occasions and has adjusted thre gas but problem still occuring. It does occassionally fire up and may do for a period of about 5 days or so and then we are back to the beginning. When it trips it tries to start about 5 times and sometimes there is a louder than normal sound when trying to fire up. A year now and still not solved the problem. After it has tripped the ignition light lock out stays on continuously.