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This sounds like a flame rectification (sensing) issue. The first thing to try is to clean the flame sensor (see attached pic for typical). You can clean off the small diameter metal rod with steel wool or an aggressive Scotch Brite pad.
Then you may need to replace the sensor. Also check the sensor connections. The sensor generates a very small current (micro amps) so the connections need to be solid. If you have a multi-meter that can read micro (not mili) amps you can check the output from the sensor while the burner flame is present. I should also mention that the sensor rod should be engulfed by the burner flame.