I am having an odd issue. Industrial demand slowing falls to negative over the course of 2-3 years. Then I save and reload the game, and Industry goes back to maximum demand, and repeat the cycle.

I have a city of >200k, proper ratio of RCI. Litterally 100+ rail connections, 100+ street connections, 4 highway connections, booming 21 X 9 airport, so connection cap should not be an issue. High education and health. Crime is fine. Residential and commercial demand is at maximum (30+ parks). Water system fine. Issues starts at seemingly any year, I've seen this from to 1920-2200. Tax rate doesn't seem to matter. I can set industrial tax to 0%, but that just slows the gradual decline, and again, everything resets to maximum demand if I just save a reload the city. Current Tax: R 7%, C: 6%, I: 3%.

Does anyone know what might be going on?

have you built a seaport? Commerical needs the airport.
and its just the dos version running dosbox.

Yes, that was the problem. I didn't have anywhere to put one though, so I lowered terrain. Also added a bunch of highway connections. Guess I forgot that rail and road connections were not enough. Thanks!

No problem!