Model Number: RGPN-07NAMGR
Brand: other
Age: 1-5 years

I have an interesting problem. When heater (which works fine) turns on, the thermostat temp will drop by three degrees, and when the heater turn off, the temp will rise by three degrees. For example: The house temp is 67, set the thermostat to 68, the heater kits on and the thermostat temp will slowly drop by 3 degrees (over a few minutes) to 64. Once the house temp reaches 68, the heater will turn off, and the house temp will slowly rise to 71.

furnace: Rheem RGPN-07NAMGR
thermostat: Honeywell 9100

The thermostat is located in a hallway away from the door to the outside.

Could this be a wiring issue the furnace? When the furnace turns on, the current / voltage is slight changed for the thermostat??