need help with ZFS

[Mikhail (Plus Plus)]

Greetings gentlemen,

I need your help with ZFS -- how I can diagnose, debug ZFS crashes, and 
if possible make it more stable. We're running raidz pool and we're 
having hard time to get it running smooth -- ZFS simply crashes as soon 
as we put some load on it.

Couple months back we've built custom server to replace our old storage 
system. New server has 7 x 1GB SATA drives, Intel Q6600 Quad core CPU, 
8GB RAM. OS: FreeBSD 7.2-RELEASE-p2 amd64, stock GENERIC kernel.

So I decided to try ZFS, and I was amazed by it's features. I've read 
ZFS wiki page and my loader.conf follows:

zen# cat loader.conf
vm.kmem_size="1536M"
vm.kmem_size_max="3072M"
vm.pmap.shpgperproc="1024"
vfs.zfs.arc_min="256M"
vfs.zfs.arc_max="384M"
vfs.zfs.vdev.cache.size="50M"
vfs.zfs.prefetch_disable="1"
kern.maxproc="20000"
zen#

and

zen# zpool status
   pool: datapool
  state: ONLINE
  scrub: none requested
config:

         NAME        STATE     READ WRITE CKSUM
         datapool     ONLINE       0     0     0
           raidz1    ONLINE       0     0     0
             ad24    ONLINE       0     0     0
             ad8     ONLINE       0     0     0
             ad18    ONLINE       0     0     0
             ad20    ONLINE       0     0     0
             ad22    ONLINE       0     0     0
             ad10    ONLINE       0     0     0
         spares
           ad26      AVAIL

errors: No known data errors
zen# zpool list
NAME                    SIZE    USED   AVAIL    CAP  HEALTH     ALTROOT
datapool                5.44T   3.54T   1.90T    65%  ONLINE     -
zen#


Problem starts as soon as we put some load on FS - e.g. last night I 
tried to rsync 1TB of different files from above ZFS pool to single hard 
drive  (partitioned UFS2), but server crashed and restarted after 
copying ~700GB of data. I also tried to stress-test it by running 100 
torrent downloads (using rtorrent), and server also crashes and restarts 
after running for about 30-40 minutes.
My guess these crashes happen due to big load on filesystem - ZFS eats 
all available memory and then server simply crashes.

Right now I'm completely lost - I can't even copy 1TB from ZFS to 
another partition..
How can I diagnose the issue? Is there anything available to make ZFS 
more stable?

Thanks,
Mikhail.