Check the air dome and hose on the lower left side of the outer tub for gunk. Also make sure the hose connection is tight at the air dome as well as at the pressure switch behind the control panel. Check the hose for any cracks/splits along its length. If no problem is found, I would replace the pressure switch
. Other possiblities are that one or more of the tub dampening straps is/are broken allowing too much motion of the tub assembly during agitation. Worn out suspension rods can also contribute to that. You can feel for the straps by reaching under the lid over the top of the tub at the sides of the cabinet. Two on each side.