I don't do it every day but sometimes I'll cut the heat for about 1/2 hour to simulate the fluctuation from the hen off the nest. It has only been about 50 degrees in my basement so it can cool down pretty fast.
 
I also read some interesting things about circadian incubation.
In nature, embryos are exposed to a daily rhythm of the hen getting on and off the nest, as opposed to artificial incubation, it helped the birds regulate daily metabolism and other physiological functions. 
 
Experiments are still ongoing but some of the results:
short periods of increased temperature on days 4-7 encouraged embryonic movement and promoted muscle and leg development
short periods of cold exposure on days 18 and 19  resulted in improved performance of month old chicks
raising the shell temperature 1 to 2 hours on days 16, 17 and 18 benefited growth performance and feed conversion.
 
What I gather from this is that imitating nature with daily cycles is better than a steady temp throughout. That doesn't mean that an incubator with irregular temperature swings is a good thing either.
http://www.pasreform.com/academy/fr...a-new-feature-of-single-stage-incubation.html
 
Good to hear. I've never had a repaired egg finish but that could have been other issues as well. If the membrane isn't broken there's no reason to believe a quality embryo won't survive a good repair unless it was contaminated with bacteria.
 
 
I think you're ok. Lethal is continuous above 104.9. The older the embryo the more capable it is of surviving temperature spikes.
 
this will answer any questions about temperature swings.
http://www.brinsea.com/customerservice/poweroff.html 
 
I still avoid high temps since a hen won't produce extremely high temps but the eggs will cool when she gets off the nest so I think high is worse than low.
 
It really helps to know what day an embryo died. So yours quit on day 9? 
 
A university of Fla study indicated the causes of dead embryos between days 7 and 17 are:
1.Improper incubator temperature, humidity, turning, ventilation.
2.Contamination.
3.Nutritional deficiencies -- riboflavin, vitamin B12, biotin, niacin, pyridoxine, pantothenic acid, phosphorus, boron, or linoleic acid.
4.Lethal genes (>30 have been described).
 
that list is from    
http://www.natureform.com/kb/newArticles/12.html
 
breakout analysis
http://www.aviagen.com/assets/Tech_...os/05HowTo5-BreakOutandAnalyseHatchDebris.pdf