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It's not possible for the heater block to change temperatures super quickly; it has a reasonable amount of thermal inertia. If by "drops drastically" and "fluctuates rapidly" you mean changes over the course of a couple of seconds, that strongly suggests the problem is actually with sensing the temperature. Maybe the thermistor isn't fully seated in the heater block, or the wires to it are loose or worn, or there's dirt or corrosion on a connector for it, etc.