What distro on what hardware, in particular, which video card)?
I haven't tested this, but you might also stop the xscreensaver daemon, then restart it from a terminal. Leave the terminal open (minimized if you wish); there might be some output to the terminal that sheds some light on what's going on behind the scenes.
Edit: Just tested. Start it with xscreensaver &. When I tested it, there was no output to the terminal under normal operation.
You might also check you log files, particularly /var/log/messages and /var/log/syslog. Again, a long shot.
Last edited by frankbell; 06-27-2012 at 07:34 PM.