From everything you have posted, it sure sounds like an ISP limitation on downloads, although I'm surprised that you have talked to Centurylink and nobody mentioned any limitations. As strange as it sounds, it could be that the people you have talked to don't know about the download limitations. A number of people in tech support are not that well trained. There are a number of people in tech support that are extremely sharp, and some not so much.
If I understand what you have done so far, you don't have a router, have changed modems, and there is nothing your computer should be doing that would limit downloads, so that only leaves your ISP as the problem.