Diagnosis
Big temp swings are usually a symptom, not the root cause. The most common culprit is a dirty or faulty temperature probe inside the cooking chamber, which makes the controller freak out and over/under-feed pellets.
How to Fix It
Locate the ambient temperature probe (it's a metal rod on the left side wall, near the top). Let the grill cool completely, then unplug it. Scrub the probe aggressively with a ball of crumpled-up aluminum foil or very fine steel wool until it's shiny metal again. Grease and soot insulate it, giving false readings.
If That Didn't Work
If cleaning the probe doesn't stabilize things, check the rubber gasket around the hopper lid. If it's torn or missing, it can let in excess air that the controller can't account for, causing surges. Also, ensure the grease management system (the chute and cup) is clear—a major grease blockage can cause flare-ups that spike the temp.
Heads Up
Don't trust the built-in probe calibration if the physical probe is filthy. Always clean it first. Also, these grills are sensitive to wind; using it on a very windy day without some shelter will almost guarantee temp instability no matter what.