DIAGNOSIS The laser-based particle sensor is either dirty or being tricked by something in your environment.
FIX Turn off and unplug the machine. Locate the small sensor inlet on the back of the unit, near the top. Use a can of compressed air (held upright) to give it a few short, gentle blasts to dislodge any dust clogging the sensor chamber. Wipe the surrounding area with a dry cloth. Plug it back in and see if the behavior calms down.
IF THAT DIDN'T WORK The sensor is extremely sensitive. It can be triggered by high humidity (like from a shower or boiling water), certain aerosols (hairspray, cooking oil mist), or even strong sunlight hitting the sensor directly. Try moving the purifier to a different location for a day to test.
HEADS UP If you smoke, vape, or burn candles/incense near the purifier, Auto mode will (correctly) ramp up to max. That's it working as designed, not malfunctioning.