I have had this symptom since day one, even with the 2005 OS and Dropbear. There must be an old thread lying around where I explained how pinging the 770 from a Linux box exactly every 4 seconds kept the link alive enough that I could ssh and scp.
Seems not all machines are quite alike in this though. Strange.
Haven't taken the time to try, but I guess the bast way to solve this is to use networking over USB as described in a Maemo Wiki Howto...