Is this a food related issue? The last couple of weeks, Coop has been horking up a little bile - maybe mid afternoon or middle of the night. No other digestive problems, and the same food I have been feeding since January.
Dogs can occasionally throw up bile if they go through long periods (ie; over night) without anything in their stomaches. A small snack before bed often cures this. Try giving Cooper a few dog biscuits or a handful of his food at night.