Strange Rheem Heater Problems

Hi All,
I am a newbie to your site as a member. I have enjoyed perusing it for quite some time. But I now have problem that I need help with as DIY kinda home owner so here goes. I have a 28 year old natural gas furnace, electronic ignition with no control board, with a very unique problem. When I call for heat, with the fan in the auto mode setting, it will not fire up and make heat. As soon as I switch the fan mode to run/on it fires up and works great. I can put the fan mode back to auto and it will run it's required cycles to meet the thermo temp setting. It will shut down and then revert to the original problem once heat is called for. If I leave the fan in the run/on position it works great and will cycle according to the heat demands of the thermo. The only problem is the fan runs at a lower RPM for the cool down cycle non stop until heat is once again called for. The fan then kicks up in RPMs to dispense heat. I have already replaced the Fan Limit control. Any ideas will be greatly appreciated. Thanks all.
Beano