If you do a search on TEMP OVERSHOOT, its been discussed.
Coal takes time to respond. If the Feed rate goes down and there is no call for heat, then it goes into Idle, then if your house cools down a few degrees, it can take a while for it to get back up to temps, and then it gets cranking and overshoots a bit. If you have the sun help heat your house, it can make the coaltrol think it doesn't need any heat and basically go down to FR=0, and it just idles along.
Once it gets going, it can take an hour or more for it to settle down, no need to unplug it.
Guess we need a bit more info of your setup, house insulated? Windows? etc...Coal Trol Setting? Min/Max,
Where is your thermostat located in relation to the stove?
Other threads to read.Thermostat overshootday night problem with hy fire 2