When i first attempted to incubate eggs i had a hard time keeping the temp constant. In small intervals over the course of 21 days the temp went down to 80 for an hour or so. Another day the temp went to 105 for another hour or so. My hatch rate was 85%. For short time periods like an hour, the temp discrepancy will not harm the eggs, as long as the temp isn't ridiculously low or high. For the 21 days, the temp can be between 98-102 degrees the entire time. The second time i hatched my temp was a constant 101 and the chicks hatched on day 20.
The thing to remember is that there is thermal mass in the eggs. Think of a baked potato, pull it out of the oven and it stay hot inside for a long time, conversely put it in a hot oven for a very short time and it is completly raw. The temp swing is not the issue, it is how long it is at that extreme. How long is a good question, I am trying to get that answered in another post.
what will happen with the higher temp. is an earlier hatch date. but watch for deformed chicks or poor hatching. A lot of the maybes are when the temp spike happens and how accurate you thermometer is. In my experience a temp of 101+ will give you a 2 day or more early hatch but with crossed beaks, bent legs and more. if your humidity is high it will be worse.
I built an incubator out of a box and today is day 19. The temperature has fluctuated quite a bit since the the house temperature has not remained steady. At times I would check and it was way too high but the chickens still seemed to grow (I candled). Out of 8, 6 eggs were alive at day 7. Have not candled since day day 14 and everything seems fine. Fingers crossed no pipping yet!