I have my probe attached directly to the heat tape with aluminum tape. I like to keep my warm end around 89-90F so I have to set the thermostat to 93F in order to reach that. Check the probe and make sure it's securely attached to your heat tape. I'd also throw another thermometer in the tub to get a second reading to compare with the acurite. Otherwise, maybe the fluctuation is due to the thermostat itself. I'm not familiar with that model.