I have an old Rheem 80. My problem is the pilot lights, main burner lights, but circulating fan does not start. After a couple minutes with no main fan, burner goes out. If I put the fan in "ON" mode instead of "Auto" and then turn on furnace, pilot lights, main burner burns and heater works. However, move the switch back to "Auto" and fan turns off followed shortly thereafter by main burner.

Any ideas?