netscape for DU running with the 4.0 libraries will periodicaly cause gnome problems. It seems to happen when netscape goes to an FTP site. at the point where you would expect to get the file save dialog box nothing happens. from this point on no new window can be opened in the desktop. It also seems that the font server dies at this point.
font server problems fixed in 6.1. This was the most likely cause of your hangs.