I have a rheem combo a/c and gas furnace with electronic ignitor. When it calls for heat sometimes it will heat then sometimes it will not.When it goes out I have to turn the thermostat off and the back on. I have a fifty-fifty chance of it relighting though it usually doean't run its full cycle before going back out.It might run for 2 minutes or not at all.or all day.If I leave it off for a while it seems to run a little longer. Sometimes.I have had the inducer motor replaced, heat exchanger,flame sensor. And gas valve replaced.The repairman said the control board is good. And everything seemed to be fine. Any help would be appreciated.I have not replaced the ignitor