silly question really... but did you check your settings (under control panel connectivity settings)? One of the options is idle times. I have found that if I haven't used my connection for a little bit of time it likes to drop it. I also found it likes to drop the connection in standby (with the cover on). The other way it likes to drop the connection is if an application made the connection (opened the connection manager). Example, have no connection and then use the web browser, but don't connect first. Close the application, connection ends.

I've ran irc on quiet channels for long periods of time with no problem.