Could heat and humidity interfere with Airport card functionality?

While humidity is not good as the moisture can condense on the colder equipment when going from an A/C’d room to the outdoors even in a case! You want to protect the system by using a sealed bag (light vacuum) so there is as little air within before exiting into the more humid environment.

As far as heat, heat can effect both the logic board solder joints and the components on it. The constant temperature change can cause micro fractures within the solder joints so the current trying to pass encounters resistance so the power won’t effectively pass. The components them selves have an ideal temp window which does get smaller as the components age.

Connectors have different risks, here tarnish and corrosion is the the biggest problem. Most of todays contacts are Palladium coated so the tarnish problem is mostly solved. Older systems like yours still has tin or copper which tarnish so the contacts surfaces may not make a clean connection. To add to that a liquid spill or just condensation can cause corrosion! And lastly age can cause the plastics to break down losing the needed compression so as the system warms the plastics become softer flexing adding to the compressive force.

While this is all generic a good eye can see most of this and one needs to look on both sides too!

If you don’t see anything obvious, I would touch up the solder joints of the circuitry that is involved referencing a good set of schematics and board view drawings which one can find online.

But if you don’t have the deeper Mac circuits skills to do this I would find someone that does. And yes, the issue still could be the AirPort board, but I would use the proper unit first, before a third party option as that adds complications!