DIAGNOSIS You're either overloading the machine or loading it in a way that creates a lopsided mass (like one heavy blanket).
FIX Load items loosely and evenly around the wash plate. Mix large and small items. Do not exceed the tub's capacity. For a single bulky item like a comforter, add a few towels to help balance the load.
IF THAT DIDN'T WORK The suspension rods that hold the inner tub might be worn or damaged. If the tub has excessive side-to-side shake when manually pushed (machine off), the suspension system needs service.
HEADS UP The machine will try to redistribute the load a few times. If it fails, it will stop with an error. Don't just restart it—rearrange the clothes first.