[rverdlm]

I understood that from your first post. I still think all the things that have been suggested are good to check. Put a good digital volt meter at the 12V panel and see if the voltage changes as the furnace runs. If your charger is weak this would show it. The furnace draws a lot of 12V current and the battery could be drawn down, recharge, draw down etc. Same with the gas if the regulator is marginal. When it shuts down does the fan continue to run? It should run after the burner shuts off to remove the heat from the plenum. If the fan continues to run (that would probably eliminate the battery voltage as an issue) and it's only the burner that is cycling, that could be normal or it could mean that the burner is getting to hot and the over temp sensor is shutting it off. Have you checked in the 2 holes outside to be sure there is no dirt etc in them that could be causing a slight restiction in the air flow?