DIAGNOSIS Yeah, this one's common. The built-in RTD (temperature probe) is either dirty, poorly positioned, or failing. It's the little metal rod that sticks into the cooking chamber from the left side.
FIX Let the grill cool. Pull out the RTD probe (it just unplugs). Clean it thoroughly with a soft cloth and some rubbing alcohol to remove any grease or carbon buildup. When reinserting it, make sure it's not touching the heat deflector or a grate—it needs to be in free air.
IF THAT DIDN'T WORK Your pellets might be the issue. Low-quality pellets with too much filler or moisture cause inconsistent burn rates, which the controller can't compensate for fast enough. Try a brand known for consistency.
HEADS UP These are big, relatively thin-walled grills. Opening the lid frequently will cause massive swings. Use a good external ambient probe for monitoring the actual cook, not the grill's reported temp.