I know purists itch when you call it a modem, so maybe it's doing more than passing bytes.

Yeah, it's definitely doing more.

At this point, I'm fairly certain that modems sold by the cableco and telcos are doing various forms of bandwidth shaping. It wouldn't surprise me if what you're running into is put in by design.

I remember reading something very recently dealing with an ISPs BW shaping models causing random weird observed side effects similar to yours. Of course, I can't remember where I saw this. If I do, I'll forward it.