Try logging on using telnet (w/ your imm account, so you don't...you know...die or anything) and see if you can make the same thing happen. You'll have to use a wall clock to count time between entering commands.

You should be able to (at least) go 300s between commands and not void. So maybe try that in Zmud and see if the connection craps out, then try the same thing in telnet.

When I get home, I'm going to see if I can pinpoint the inactivity threshold at which my connection craps out. Will let you know.