Whenever I try to download a large file from Dropbox, the download always fails, usually after a large amount has already been downloaded. Try to restart and pick up where I left off, but that always fails as well, so have to start all over...and eat more data.
I used to use Free Download Manager many years ago when I had one of the legacy, daily refill plans. It worked well then, though I can't speak for how good it is today. Still, it might be worth a try. If it's still the same it allows you to pick up from where it left off, meaning you wouldn't have to start all over again with another attempt.
Granted, that doesn't address the problem itself, but it's a possible workaround.
Haven't heard back from you in a while, so I hope Gabe's suggestion helped! Otherwise, you could also try the download during the Bonus Zone.