Since your using one single piece, it's strange that you would see that fluctuation. Was it always like this? How often have you been monitoring the temps?

I say it would be flexwatt gone bad, and the heating elements aren't evenly heating.

When you take your temps, are you testing in the tub on the plastic or on the shelf directly on the heat tape? Or, when you use the temp gun could you be pointing it between the elements on the laminate by mistake?

Could it be your gun went bad?

Also does your thermostat have power modes such as trickle, or proportionate etc..?

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