The main burner of my Magic Chef Ultra II furnace will not light normally. The furnace has an electronic ignition. Its blower, electronic igniter and pilot burner seem to be functioning normally, but the main burner won't fire up unless I blow on the pilot flame several times. When I do blow on the pilot flame the furance continues running until my homes temperature reaches the temperature my thermostat is set at. Sometimes it continues cycling normally for a few hours and sometimes it doesn't ( only the blower and pilot burner come on). This summer I replaced its thermostat and a couple of days ago I install a new flame sensor. When I replaced its flame sensor it seemed to cycle properly for about 24 hours, but then it went back to only the blower and pilot burner coming on. Any input regarding what is causing the main burner to not ignite would be greatly appreciated. Thank you. Sampson12