Diagnosis
That's a classic sign of a clogged burner tube. Grease, spider webs, or food debris is blocking some of the ports, forcing the gas to find only a few exits, which creates a high-pressure, sooty yellow flame.
How to Fix It
Turn off the gas and disconnect the propane tank. Remove the grates and flavorizer bars to expose the burners. The problematic burner is held by a couple of screws. Take it out. Use a grill brush, a straightened wire coat hanger, or a specific burner cleaning tool to clear out every single port along the tube. Blow through it to dislodge any loose debris before reinstalling.
If That Didn't Work
The issue might be a faulty control valve for that specific burner. If the burner tube is clean but the flame pattern is still erratic and only on that one, the valve orifice (the small brass piece the gas comes through) could be damaged. You'd likely need to replace the entire valve assembly for that burner.
Heads Up
Regularly cleaning your burner tubes at the start and end of the grilling season is the best prevention for this. Spiders love to nest in those dark, cozy tubes over the winter.