First thing to do: make sure you're calm. A temporary temperature drop is not nearly as bad as a temp spike. When you're calm, you can avoid making hasty temperature adjustments on the fly.
Second: are you relying on the incubator's built in thermometer, or do you have several additional thermometers inside? The built-in thermometers on foam incubators are notoriously inaccurate. Always always use an analog thermometer along with a digital.
Third: I've found that messing too much with humidity in foam incubators has a tendency to create temperature swings. I like to hatch "dry' (you can search any number of informative threads on this incubating technique, or talk to BYC user @AmyLynn2374 ), in other words, don't add any water until day 18 unless the humidity goes below 20 percent, and bump the humidity up to 70-75% on day 18-19. You would also monitor the air cells when candling.
Fourth: You can SLOWLY adjust the temperature up, and allow it to stabilize between adjustments. This is important to prevent a temp spike which could be deadly.
Hope this helps! You can also join several informative threads on here to talk with hatchers more experienced than me
Good luck with your hatch!
