When you have an outfire, is the coal completelty burnt up? so you have ashes all the way to the bottom of the pot?? or is the pot full of fresh coal, and the layer of ash very thin?
When you said you checked your draft and 'it seemed ok' , how did you check the draft?? Do you have a barometric damper installed in the flue pipe??
If you have too much draft, it could be pulling too much air through the coal when it is not stoking.. this could increase the heat from the fire, and keep the water temps high..
Use a manometer to check and set the barometric damper to the corrrect setting,, about .04 to .06" wc if I remember correctly.
Somehow the fire is making too much heat at idle. The above scenario is all I can think of.
Hope this helps