Diagnosis
This is usually caused by a dirty temperature probe (RTD) giving false readings, or ash buildup around the fire pot disrupting airflow.
How to Fix It
Clean the RTD probe. Unplug the grill, let it cool, remove the grates and heat diffuser. Use a dry scouring pad or fine steel wool to scrub the silver probe rod on the left wall until it's shiny.
If That Didn't Work
Perform a deep vacuum of the fire pot and the entire burn chamber. Ash touching the probe can insulate it, and excess ash in the pot chokes the flame.
Heads Up
Temperature swings are also normal when you first open the lid—the controller will overfeed pellets to recover the heat, causing a spike. Keep lid openings brief.