To note, even in nature there are Temp swings when the hen leaves the nest to eat or temps rise above 100, 101, 103......
Temporary temps close or around the optimum temp will not have that much effect other than slowing or increasing the development process. Long exposure will have an effect, usually some deformity.....but that is not a rule. Mother Nature usually has seen to it that some variations of conditions are allowed without consequences.
Quail Jailers advice is good. Circulating the air will help keep a consistent temp and help distribute the hot air to the cold spots, thus causing the incubator to equalize the temps.
However, I have read that Pheasant do not do as well in circulated air incubators. I don't see why not?