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The biggest causes for ignition fault are:
Low gas pressure
dirt flame sensor
Since it happens sometimes and not others I would rule out ignitor. I would use steel wool on all the ignition/metal parts at burner. as well, clean/blow out the burner. As far as pressure, you would need to call for service
Soeme of these also have a screen for the intake air that can become plugged/dirty which inhibits air flow.