Pyrox vulcan wall re-ignition problems

When the heater button is pressed, this ignites first left outlet then across to the middle then the right outlet. Every day the gas heater cuts out, and the fan stops. Then the re-ignition comes on, (it sounds stuck), with buzzing sound, sometimes it ignites, sometimes not. This can go on for half an hour, til we turn it off. If the buzzing/ignition goes on, without being turned off, a minor explosion results. Any ideas? An expert has been twice but hasn't been able to fix problem.