I have an older Rheem furnace for my back garage. The ignitor was bad so I replaced that, but now the burner will kick on for about 20 seconds or so then shut off. Then the fan will turn on for awhile and shut off. Then maybe again the burners will do the same thing. this goes on for awhile and every now and then it does cycle correctly. Any suggestions would be appreciated- Ken. kllh58@yahoo.com