DIAGNOSIS Large, sustained swings usually point to a dirty temperature probe (RTD) giving the controller bad information, or the controller's P-setting needing adjustment for your environment or pellet type.
FIX First, clean the RTD probe. Let the grill cool. The probe is a thin, shiny metal rod on the left interior wall. Use a soft brass brush or a dry Scotch-Brite pad to gently scrub all sides of the probe until it's shiny. Don't bend it. Ash buildup acts as an insulator, causing false low readings that make the grill over-fire.
IF THAT DIDN'T WORK You can adjust the controller's feed rate (the P-setting). From the main screen, press and hold the ignition button for 5 seconds to enter the advanced menu. Scroll to 'P SET'. Increase the number (try going from the default to 1 higher) to slow the pellet feed rate if temps are running high, or decrease it to feed more if temps are low. Make small changes, one increment at a time.
HEADS UP Don't rely on a separate oven thermometer as the sole truth unless you've verified its accuracy. The Traeger's reading is from a single point. Some fluctuation (within 15-25 degrees) is normal, especially in windy or cold conditions, as the controller works to maintain an average.