Powermax 155x Backfires after running for 30-60mins?

I have had the boiler serviced and a part replaced. When the boiler is first switched on the usual ignition is fine but once it has run for 30-60mins and the ignition needs to kick back in, it Pops/Backfires. I take it as the Gas is running through and the Ignition has a delay and the chamber fills with Gas and when the element heats up the ignition has greater volume of Gas than required, hence the Pop/Backfire. What it the fault and what needs replacing?