Furnace will operate intermittantly when thermostat is set at 90 degrees will not operate when thermostat is set at a lower setting. Sometimes this does not work either. What are the possibilities for this problem?

Is this a round thermostat with a mercury bubble inside?

No, this is an electronic thermostat

Sorry, I was hoping for a mercury switch that must be 100% level. Can you tell us brand and model of the tstat