The other thing that seems to vary wildly is T-Mobile's inactivity timeout length. In some locations, I can sit and read a web page for twenty minutes without being disconnected; others, five minutes of inactivity is enough to get me disconnected. I assume it has something to do with cell site load.

Do you get disconnected while actively using the connection? Try downloading a large-ish file (say, 15 mb or so) and see if you get disconnected in the middle.