You can use a fan while inside to help keep the drone cool. This helps when doing firmware updates or checking out the Litchi/DJI app settings that don’t show up unless you are connected to the drone.
“…why (sometimes) the reception bars jump from “fine” (3bar) to “oh no” (0 bars) and back and forth.”
Microwave Links are supposed to be at least one reason for this behavior. If you fly through the transmission signal, it will seriously affect your RC signal. If you are able to keep flying, the signal will come back after you get past the microwave towers effect. However this can be risky and DJI advises to avoid RF intensive areas that can affect the drones flight.