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What you describe sounds like a flame sensor problem. If you're OK working in the combustion chamber then you can try cleaning off the metal flame sensor rod (see attached pic for similar, yours won't have 90 degree bend). Remove the oxidation on the rod with fine sand paper and take care to not move it out of place. It's placement in relation to the burner is critical. Also vacuum out any rust flakes in the chamber since a rust flake can ground out the rod.