The only thing that happens is that after 15 minutes of inactivity 1) VDU and 2) the disks are switched off

Seems to me that could cause a synch failure. Not so much the display, but the disks. Try turning off the disk spindown feature and see if your problems go away. It's unlikely Netscape or interrups with the ISDN adapter.

-- Tony Fabris -- Empeg #144 --
Caution: Do not look into laser with remaining good eye.