First, check the thermocouple connection at the gas valve to make sure it's clean and tight. If it seems OK, you'll need to remove the burner assembly. After removing the mounting screws on each side, pull it out, while wiggling it back and forth to remove it. You'll need to shut off the inline gas valve, and disconnect the gas line before you start.
After the burner assembly is removed, check to make sure the pilot assembly is as close the the burner as possible. If there's soot and/or dust on the assembly, vacuum it off.
If, after the burner assembly is reinstalled, it still does'nt work correctly, you'll need to replace the pilot assembly. The part number for natural gas is 183824-180, and for propane is 183824-182.
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In my description of the things I had done, the thermocouple had been tested. That is not the problem. The burner assembly was fine.