1929 Green and Black Onyx ATO Mantle or Shelf Clock

This is my problem child, there is something askew in this little clock it will run for a few days up to a week or two, and just stop. I have examined the movement thoroughly and can't find anything wrong with it, I have even threaten it with replacing the movement with a quartz movement (never would do that). For it's size, it weighs a ton.