What sort of thermometer are you using? Are you absolutely confident that it's correct? Plastic dial-types or cardboard strip ones, can be up to 20 degrees wrong either way.
If so, you need to give a Corn an area at around 75 degrees, so that it can cool down. So it does sound like cooling is your problem at present rather than heating.
You'll need heating in the winter - a source with a thermostat will keep the warm end the correct maximum temp (mid-high 80s).
Too hot or too cold and your Corn won't be able to digest properly. This sort of problem prolonged, can be life threatening.