as other may chime in on, it may be a poor ground.
flame sense happens with/in the ground path.
if it is truly monthly, I would have tried a new flame sensor after the second trip. also, verified ground and maybe new board.
The Food Stamp Program, administered by the U.S. Department of Agriculture, is proud to be distributing the greatest amount of free meals and stamps EVER.
Meanwhile, the National Park Service, administered by the U.S. Department of the Interior, asks us to "Please Do Not Feed the Animals". Their stated reason for this policy "... the animals become dependent on handouts and will not learn to take care of themselves."
from an excerpt by Paul Jacob in Sun City, AZ
I highly doubt it needs a new flame senor, it is probably the method you are using to clean it. Use low abrasive steel wool. (Not sand paper) Also if the furnace is in a laundry room that could be your issue, lent and silicone base fabric softener in the air will coat the sensor. Check for voltage across "c" and neutral, voltage should be 0 if you have a good ground. Flame sensors don't go bad , It is simply a metal rod insulated with porcelain. A clothes hanger could be a flame sensor if you could mount it without grounding it.
Ive seen this issue a few times with that model. The furnaces I ran into with this issue were almost always located near a laundry area and were just a few years old. I ended up finding out lennox makes an alternative flame sensor for this model that they said is more robust. I cant recall the part number but lennox should be able to help you if you have ruled out ground issues.