I had issues with my 680 maintaining temps at the low end. Customer service talked me through a programming change on the phone and it's been solid ever since.
If the pellets have absorbed too much moisture from the atmosphere you will have the temperature issues. I find this happens when I use a partially opened old bag I forgot about. Use a shop vac to clean out the auger.
Also I had issues with it maintaining temps at low end. Call rec text...