Temperature Fluxes - How much is okay?

[Teal]

So I've got my Kingsnake eggs in a styrofoam cooler, in perlite, with a heat pad at the bottom of the cooler and the little container with the eggs on a little shelf above the heat pad.

But, because the temperature in my room fluxes a lot throughout the day/night, the temperature in the incubator fluxes too.

The fluxes have mostly been within what I've read to be the acceptable incubation range for Kingsnakes (75-85F)... but there's been a time or two when I've caught it at 70, and then at 90

So, what I'm wondering is... How much fluxing can the eggs handle? Did those few times at 90F fry my eggs? Will the constant temperature change cause them to not develop? I'm trying to figure out a way to stabilize the temperature in there more, of course... but with the fluxes for the past week, might I already have doomed them?