>Doctor has no suggestions on why<
That's because doctors are like automotive mechanics; most of them are idiots. (Don't confuse a surgeon, with a doctor. Surgeons are specialized wizards, practicing a magic craft. By the way, written by the top heart surgeon in the US: “Woond heeled well.” Again, written by the top heart surgeon in the United States! I mentioned it because I wanted to stress that critical person, picking on small details when I say most doctors are idiots.)
There is strong evidence that your problem could relate to bacteria bloom in the gut. Based on your clockwork-timing, I would bet serious money that it's a gut-bacteria issue. Even so, trying to explain something like this to a doctor would be like trying to explain witchcraft.
Do your homework, and here's a starting point:
https://chriskresser.com/more-evidence- ... vergrowth/
Mine was the opposite. I used to get “acid-re flux ” for decades. I told them that it wasn't re-flux because acid is neutralized by alkaline, aka antacids. This is junior-high-school Science 101, the reason I'm less than impressed with their “wisdom.”
Nothing would kill the “acid-re flux.” I finally discovered it that certain spice(s) exterminated good bacteria in my stomach. My digestive system would shut down. Eliminate the spice(s) and problem solved
Spices were not for flavor; they were used to make rotten meat playable. My ancestors (one of whom was a Passamaquoddy squaw) lived in a small isolated, European-American community, starting about 400 years ago. My ancestors consumed mostly fresh seafood, and whatever living creature they were fortunate enough to capture. So unlike 99.9% of the American population, I'm not genetically programmed to eat rotten meat, spiked with spices so you can choke it down. Hots? It's not a case of not liking them. I cannot get them in my mouth, let alone wallow them! I'm not a picky eater, so I can eat food with seasoning, but if it's up to me, a little salt, a sprinkle of pepper, and it's very tasty.
Anyway, I don't know what spice(s) they are, but many restaurants use them, almost a mainstay. But in the last couple of decades, I have been able to detect the “trigger” spices the moment I taste the food. I suspect the spice(s) kills digestive bacteria because my digestion system shuts down.
In your case: It's probably a bacteria bloom, and the clockwork timing matches bacteria cycles. Spores and bacteria, boom year round, depending on day-night cycles. There could be a specific fall bacteria that causes a bloom of bad bacteria in your stomach.
You're going to have to do some serious homework because you won't find a doctor who can fathom such a concept. I skimmed a few paragraphs. It's Chinese to me, sorry; I just understand basic science, a subject doctors don't understand. But that was a 3 second google hit, so you might find lots of information because everyone in the world isn't an idiot, just doctors. Maybe between your research, and good medicine here, you can find a solution. If you could determine the bacteria type in the colony, the cure would be one swallow away.