Edgestar 12000 heating problem

Dear Guru:

After I set the heating thermostat set point ("ST" on the unit) (say 75 degrees), my heater will run if the room temp ("RT" on the unit) is lower than 75 degrees. However, after the heater runs and reaches 75 degrees, the "RT" will stay at a temp higher than 75, sometimes in the 80 to 83 range. In turn, the heater will not come back on because it thinks the room temp is higher than 75. I know the room temp is not higher than 75 because I have a separate thermometer in the room to measure the room temp.

Thank you.