Based on my experience using Guru ATC many times.
1. You say you "started it" when the pit temp came up to 250 deg. Don't do that, it should be on as you place the lit on top of the fuel. Let the ATC bring it up to temp from the very beginning. I.E. from 0 degrees the ATC should be controlling the fire, including the start. Very important. I think this is the key to the experience you had.
Other stuff to consider:
2. You need to start with less lit. I would use the minimum. I use lump and take some care to see that it's stacked well to eliminate air pockets. I use 10-15 lit briquettes arranged on top. Let the ATC do the work of bringing the fire up to temp. Next cook start with 10. It might take a little longer to come up to temp, but the ATC will do the work.
3. Bottom vents all closed, top vent open.
4. The fan will come on from the start. It will typically stay "cycled on" longer during the start.
5. During the cook, it "learns" and cycles for shorter periods. Sometimes only seconds at a time. This is normal.
6. Very important - regardless of ATC or not an over under of 25 degrees is meaningless. Your indoor oven is most likely off this much or more and you aren't checking it every 15 minutes
As long as the ATC is trying to adjust for over/under by cycling the fan it's working properly IMHO. You still have to have some knowledge of fire control, the ATC alone doesn't eliminate that skill.
Do you have an air leak/out of round condition? If you think this is possible, place a "seal" off foil between the bottom and mid section joint to see if it makes a difference.
I typically see very minor swings in temp with the ATC as long as any/all of the above are covered. With my ATC it will cycle/learn as well based on the food temp, pit temp, and whether the lid has been just opened. Pretty sure the Stoker does that too. So just be aware that fan time is dependendant on more than just one factor.
