To tell the difference between discoloration of the metal caused by high heat, and product attaching to the metal part, try to erase the color with a common pencil eraser. Discoloration caused by high heat will not erase off. Some of the product is sticking to the metal surfaces restricting their movement. Heat is the main cause. The product pressure has dropped.
Many fluids solidify with a drop in pressure.
Paint is a good example of this. The paint solidifies when the solvents evaporate. Air or oxygen is getting into the system. Through the stuffing box. The product was not deaerated.