2000 Honda CRV Temperature Gauge Rises at Idle?

When my Mom comes to a stop sign on a hill near our house, the temperature gauge shoots up into H. If she turns the heater on the gauge drops back down to about halfway. We replaced the thermostat yesterday and added coolant to both the radiator and overflow. She drove the car today and stopped at the same Stop sign. Temperature is still shooting up. Checked overflow and radiator. Both still have adequate coolant. Need help determining cause of temperature issue. It only happens when the car is sitting at idle on a hill.

Any help or recommendations would be greatly appreciated. We want to avoid seeing an mechanic.

Thanks in advance!

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