This doesn't appear to be a proximity problem as far as I can tell.

Also, an additional data point: This only seems to be an issue when the large file download is coming very fast, i.e., it's saturating the bandwidth.

For example, right now I'm downloading a huge file from a web site, but everything is working fine because the download is trickling in at only 87k per second. It's only when I have something that's coming down the wireless pipe very very fast that there seems to be an issue.

Might that be a clue? Is there something that a wireless connection does at high bandwidth that it doesn't do at low bandwidth? Something that kicks in and out instantly when the demand changes?