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Is it a power vent model or atmospheric discharge?
Also whose' the manufacturer.
You mentioned the pilot goes out at least once a month,is this accurate or is it failing more frequently.

The newer models come with a "fvr" sensor on the milli volt end it called Flammable Vapor Resistor,it will open the circuit when the ohm reading goes above 75k.(from flammable vapors are detected).
These are normally installed on power vent units with an HSI ignition.
Also called glow plugs or Hot Surface Igniters.
As these are a bit tricky to reset it's a multi step process,
or you have a standing pilot, the thermocouple may not be positioned properly over the pilot or you my have flame rod (instead) I've seen this manufacturing defect a 100 times before.
I honestly do not think you have a combustion issue.
Posted from my iPhone/Mobile device