You will find the quality of coal does make a difference.. I used Reading for awhile and was astounded at the difference when I started using UAE and Superior coal.. as close as you are to the good stuff, I'd burn nothing but the best..
You said you have .04" draft when the stoker is running,, what is the draft while the stoker is off and the fire is idling?? Is this house really tight?? try opening a window and see if it allows more draft. If so you need to pipe an outside air source to the fan inlet..
Like Stoker man said,, if the water temp is too high, the timer is kept from running the stoker,, in the summer you don't need really hot water for heating, only for DHW,,, try setting the aquastat down to 150-160* and see if this gives you more margin to the high limit temp for the timer.. Sometimes boilers just get hotter and hotter on the 'keep fire' burns, without any DHW being used,,
Have you verified that the timer is running the stoker every 30 minutes?? Or is that just what it is set for?
When the fire is out,, do you have a depression in the firepot, and completely burnt up coal?? or do you have a level layer of coal that is partially burnt?? if you have the latter, you might try hooking up a small fan,, about like a hair dryer in size, and blow this at the inlet to the fan, this will add a little airflow through the fire... and keep it burning.
Hope some of the above helps..