DIAGNOSIS This is almost always a sign of a clogged burner or a problem with the air shutter. Yellow flame means incomplete combustion, often from debris blocking the gas/air mix.
FIX Turn off the gas and let the grill cool completely. Remove the grates and flavorizer bars. Inspect the long, tubular burner. You'll see a line of small holes (ports) along its length. Use a stiff nylon brush to clean the outside of the burner. Then, take a thin wire (a paperclip works) and gently poke it into each port to dislodge any spider webs, insect nests, or grease clogs. Next, look at the end of the burner where the gas hose connects—there's a small air shutter (a slotted metal disc). Make sure it's open about halfway; you can adjust it by turning it slightly.
IF THAT DIDN'T WORK The regulator itself might be failing and not delivering consistent pressure. The only reliable test is to try a different, known-good propane tank and hose/regulator assembly. If the problem goes away with a different tank setup, you need a new regulator.
HEADS UP Never operate the grill with a predominantly yellow, lazy flame. It produces excessive carbon monoxide and soots up your grill. Always aim for a steady, blue flame with a slight yellow tip.