Hi Brian,
I think it might be something to do with your network.
I regularly serve large files from NetTalk - all the webinars for example are served from a NetTalk server, and they are from around 80 megs and up. Granted that's not SSL, but I don't think the SSL part should matter.
And I don't _think_ the session timeout should play any part - that's an intriguing question I'll think about. But that's easy to test - simply make the timeout longer on the WebServer settings. (it's in hours:minutes there).
Cheers
Bruce