Outfires are caused by lack of coal, lack of air, or both.
Little to no chimney draft can cause outfires, and plenty of other issues. If you have never checked the draft with a draft gauge, at least check to make sure that the flue pipe is clean, there are no gaps in the pipe, the clean out door on the chimney is tightly closed, and that the flame of a match gets pulled through the hole in the fire door when the stoker is running. A draft gauge (or manometer) is needed to fine tune it, but for now just verify that the firebox is negative compared to the room when the stoker is running.
Also, if the load on the boiler is very light, and you have the end switch of the zone relay connected to the aquastat on the EFM, it will ramp up to the high limit and likely skip timer cycles. If the house only has radiant heat, a simple solution would be to wire the circulator(s) to run independently of the stoker, and let the aquastat control the boiler temperature via the low limit.
An intermittent obstruction at the opening of the auger can also cause outfires. I battled outfires for a month one time and was going crazy...then someone told me to empty the bin and see if I found anything. I found a piece of nut coal that was worn nearly as round as a pin pong ball...it had been rolling around the end of the auger, and prevented the coal from feeding properly.
Regardless of the outfires, you should be controlling the temperature of the radiant zones with a mixing valve.