I have the exact same problem in Slack81. Annoying, isn't it? I had chalked it up to the fact that I was using a USB optical mouse. I can pretty much make it happen if I run xmms, although mozilla has also knocked it out a couple of times. I have tried unloading/reloading all USB modules, unplugging/inserting the USB connector for the mouse, restarting X and all combinations of the above and still the only way to 'fix' it is to reboot.
I was going to try the latest XFree patches from slack, but figured I'd just wait for 9.0
Anybody have any insight to this?
The only hardware I run slack/xfree on is a Via MicroATX board so I don't know if it's hardware specific.