Temp Gauge

I have a ’98 Honda Civic EX. Recently my temperature gauge has been moving up to around three fourths mark. I’ve changed the thermostat and radiator cap. The radiator fan works, I jumped it, check voltage while the car was on, and I finally hear it today while I was sitting at a stop light. Here’s the weird part, my temp gauge will stay around normal when I have the heater controls on hot, heater off, and the temp gauge will jump up to three fourths when I move the heater gauge to cold, still have the heater off. Car doesn’t heat up when just sitting, just when I’m driving down the highway.