Baxi Combination HE80 ignites with an explosion?

4-5 years old. Whenever the boiler ignites, there is a large explosion. Several engineers under a maintenance contract have had a look, the igniter has been changed, the insulation sleeve was suspect and trimmed. The engineers leave it in a better state but it always degenerates. The next solution is to change the circuit board but this will be at my expense and I really would like to see a bit of fault finding to identify the problem instead of changing parts. Any ideas?