Well, it seems that's a very insightful piece of intuition you had here ! There does appear to be some radical power management going on in the radio subsystem. Apparently it's even done while on mains power, although maybe less aggressively than when on battery : with the AC adapter I managed to connect from XP with Putty on first try, and only had the first freeze later on when in MC. I also verified that (re)loading a web page in Opera unlocked the ssh session immediately.

So my experience verifies your guess :

- absence of WLAN traffic for a very short period of time (seconds ?) will put the 770's network subsystem in "sleep mode"
- outgoing activity from the 770 wakes it up immediately and transparently
- incoming activity has a harder time waking it up (maybe a poll at fixed intervals ?)

Fortunately, it also seems that incoming activity on a live interface prevents it from going asleep (which sort of makes sense :-). So there is a very simple workaround to ensure comfortable ssh sessions without annoying pauses, under Windows or Unix :

- launch a console windows and type : ping -t xx.xx.xx.xx (IP of 770)
- "activate" the Nokia's WLAN if necessary by loading a web page or such
- verify that the ping commands are successful and stay that way
- launch ssh session. Mine has been running idle for more than an hour now, with no freezes

Thanks for providing the solution ! Now I must verify that my wild goose chase with IP subnet masks yesterday was total crap and coincidence, as it must surely be :-)