I compiled a kernel after yesterday's sup (including disksubr.c,v
1.30.4.1), and my disk problems are fixed. Thanks!

Disklabel works fine all day long, also installed a release on another
hard drive (under X), then dumped restored /usr to another partition.
I then booted after creating a large MacOS ramdisk in order to limit
memory and force use of the swaps on both drives. Root is now on the
Seagate ST3600N, /usr and /home on a Seagate ST39173N, 128M of swap on
each drive, ram limited to about 30M (Quadra 840AV). This is similar
to the configuration I had before everything went to heck last August.
Now performance degrades gracefully under heavy load; there's only a
slight pause as an application swaps back in when it's window is
mapped.