Tyan S2885 Thunder K8W lockups

[NAKATA Maho]

Hello
sometimes my opteron box locks up recently,
it seems that there are some discussion about this issue
on the list.

o Opteron 250x2
o Tyan S2885 K8W
  on board SATA/IEEE1394/USB2/GbE/AC97
o Total 10G of ram (512Mx4+2Gx4) : tested with memtest+ 1.6
o BIOS 06/10/05 TYAN  Thunder K8W (S2885) V2.05
o FreeBSD 5.4-RELEASE

How to lockup:
# dd if=/dev/mem of=/dev/null bs=1024k
about 50 seconds later it lockups. 100% reproducible.
* Both Optimal/Failsafe setting in BIOS
* Both MTRR Mapping Continuous/Disabled
* set hw.physmem="4G" doesn't help

workaround:
on board GbE (bge0) and SATA disabled.

any hints or comments?
-- NAKATA, Maho (maho at FreeBSD.org)