Thank you for making this experiment.
"Could a [varying time expended] in a [download result in] almost exactly the same [truncated] size each time?"
This would have me believe that there is no time limit getting hit. So, need to look elsewhere.
"2 different PCs"
Are the computers both using Chrome? (Yes, I'm really reaching for a suspect.)
I am finding a few posts in other boards complaining about a download stopping after 500MB. Reading them now.
From a Wordpress board, "balloonartonline" writes: