If it's a bad gasket that makes the cap vent fuel vapors on hot days, why does it happen with both new and old caps?

The spring on my gas cap works exactly the opposite way to what you are describing. It is set up to allow the gasketed plate to push away from the filler neck under pressure from inside the tank - if you push on the cap, there is give, even when fully tight. When I said it's like a radiator cap, I meant exactly like a radiator cap.

It's also rather difficult to buy into the idea that the pressure differential from condensation in the cold is high enough to destroy a tank, but the pressure differential from vaporization is inconsequential, especially with a substance with as a high a vapor pressure as gasoline.

I have read the article you linked, but it's just something some guy I've never heard of, with a background that says nothing to his automotive expertise, wrote on the internet. It might be bible truth, it might be so much blather. Honestly, I am WAY more likely to just take your word for it, than I am to accept this other random guy's opinion. It doesn't hold much weight with me, compared to the evidence of my senses.

-Kle.