About frequent disconnects: I don't see them when tethering from N810 to N900. I only started today, and had two sessions: One disconnected after 40 minutes, and the other is still running after 2h45m. I did not transfer a lot of data, though.

The problem might be related to this bug: https://bugs.maemo.org/show_bug.cgi?id=6942

See if you can add any information.

If it disconnects exactly every 10 minutes it might be related to wifi (10m is the default search interval), try to disable wifi and see if it changes anything.

There is a simple way to find if the problem is in the BT, or after the BT in the PPP server: on the host computer open a terminal and connect to the BT serial port (on Linux it would be minicom and /dev/rfcomm0, I don't know how it is done in Windows or Mac). Then run the command ati. Keep running it every minute or two, and see if it still disconnects after a short time.