It is likely due to a dirty or defective flame sensor.
Cleaning the flam1e sensor (ignition sensor) in the lennox 80MGF series furnace with 200 grid sand paper restored full functinality.
The heat exchange orfices need to come out but disassembly is easy. Turn off the main furnace power switch or breaker.
After taking the gas orfices out, the long metal bar shows the ignition and heat sensor probes.
After a little manipulation to get the assembly out it is easy to clean all probes with sand paper untill the gray powder surface deposits are removed.
Reassemble and turn the furnace power back on.
You will have full control again and the furnace will stop cycling on and off.