This same thing happens when you allow rust to penetrate behind automobile paint and you then notice that the paint is peeling off in sheets. The elastomer will usually swell up and get soft. When an elastomer shrinks and gets hard it is almost always evidence of excessive heat. Metal components will develop pits and an overall dull appearance. Be sure to vent vertical pumps.
Air trapped in the stuffing box is a good insulator.
See if the operator is running the pump with a restricted discharge. In addition to deflecting the shaft it can cause a severe heat rise in the pump. The control valve may be stuck in the throttled position. Try to use a recirculating line from the bottom of the stuffing box to the suction side of the pump. This is practical in almost any application other than when we are pumping a product close to its vapor point and there would be a danger of vaporizing the product in the stuffing box.