I was talking to a tech recently, and he said he had similar problems. Took him a long time to figure it out, but the thermocouple had been dislodged slightly and was not sensing the system temperature properly. ALl he had to do was reseat it, and it worked fine. Might be something as simple as this. You can often find the specs on the thermocouple in the aquastat. It has a predictable resistance based on temperature. You can check it to verify the accuracy, and, to see what temperature it thinks it is seeing.