My G336N fireplace is 15+ years old and will not stay lit. The constant flame pilot is working fine. Lately it does not light immediatly once the wall switch is turned on. Then once it does light it will not stay lit. The amount of time it stays lit varies. In looking through literature from the original owner I see the thermogenerator may have been replaced. Is this the part that controls the flow of gas to the burner? How can I get a repair manual for the unit? And how can I troubleshoot the unit?