Good stuff here......although I don't know the answer (and won't pretend to either), but if I had to guess, I would think it would have something to do with heat. If it runs for 20 minutes, and won't start after that, sounds like "something" has heated up and needs to cool before the engine will start. I would doubt it would be air in the fuel line, as it runs fine for 20+ minutes.....I would think if it was air in the fuel line you would see signs well before 20 minutes.
The black gas tank thing does sound a bit strange, but if painting it another color worked, then by all means try it. I wouldn't scrape the existing paint off, just paint over it with silver.
I'd love to see where this ends up as it seems to have a few people here stumped.