Wow. Although I do like my forced air picture window hovabators (I love watching the little ones hatching), I like my LG's to incubate hand turned chicken eggs. I got mine for free and they had been used before, but only once. During the winter time, or when it is below 60 in my room (which is rare) I will wrap the back of the LG (leaving the large vent holes open) in a small blanket. The first time I plugged mine up, it took me about 6 hours to get them stabalized. I've never had a temp spike, or a dip. They vary by 1 degree daily. During the day they are at 100 degrees, and at night they are at 99. Mine are forced air though, so that may help stabalize them a little quicker.
The only thing I don't like about mine is that it is hard to keep the humidity high enough to incubate duck eggs in them.
Now I have two still air hovabators, and I'm still getting used to needing to run them at 101-102 degrees!
I haven't officially turned any of my incubators off yet this year, but I have washed and cleaned them between incubations/hatches. I've never had a problem with getting any of them to stabalize.
My only advice, let them run, and don't fiddle with them too much. If you're changing the temp more than once every three hours and you're not accustomed to the little knob, you could be changing it too much without knowing it. I also keep all of my vents open during incubation and hatching. Don't know if that makes a difference or not, but it's worth a try!
Emily in NC