Furnace cycles on and off
At the opposite side of the burners from the igniter there is a small metal probe/rod situated where the flame would blow across it during operation. This rod is for flame verification so the gas valve will know that the gas is being ignited. Often that probe will get carbon built up on it which will cause the furnace to light but then abruptly shut off because "the probe doesn't know there is a flame there". A fine grit sandpaper works well to restore it's conductive ability. However, if the furnace runs for a while before it shuts off you could very well have an obstruction in the vent pipe or an obstructed indoor coil. If cleaning the flamerod doesn't remedy the problem, I strongly recommend a qualified serviceperson look at it! The furnace could well be turning itself off because something is getting too hot and coming back on when it cools. Good luck.
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