I'm working with a Heil gas furnace model # NOG5050BEA2. The unit works fine for a few days and then the burner goes out and the blower motor stops but the evacuation motor keeps going and the thermostat is calling for heat. The burner will not relight unless the thermostat is lowered to drop the call for heat. After a short while the unit will work fine again for an intermittent time. Sometimes days and then sometimes just hours. The burner has been adjusted and the thermostat has been replaced. Also replaced the rollout limit switch. Have improved the air to unit. Any suggestions would be appreciated...