I have a B&W G3 tower (running 9.1 and 10.04) that this was happening to, when shutting down in OS 9 mode. Before I got it to stop, I tried a lot of things, including getting rid of the RAM disk I was using for web-browsing cache and disabling my Palm serial watcher software that started on boot (along with the Keyspan serial adapter extension it uses).
I don't know what fixed it, but if either of those sounds familiar, let me know.
(Now if I can just get the machine to boot cold to the OS 9 disk, I'd be happy -- it always boots into X if it's been powered down. I think it's a master/slave IDE issue.)
[ 08-25-2001: Message edited by: welborn ]