My thought on this is as the blades are put under load of the heavier grass the spindle is allowing enough movement of the pulley to throw the belt. So checking for any movement of the spindle through the rotation could be the problem. Checking using the blades will give the leverage. So if the bearing even have one spot that is worn enough it would throw the belt. Pushing on one end of the blade towards the deck as it is rotated may find problem. Good luck finding what is causing this.