Regular reboots with > 2G memory

[Brad L. Chisholm]

I've been working on setting up an Opteron system based upon the 
Tyan Thunder K8S (2880) motherboard.  I saw the reference David
O'Brien made earlier regarding possible issues with this motherboard, 
and I am wondering if I am being bitten by one of them.

Here's my scenario:

   Tyan Thunder K8S (2880) motherboard
   2 Opteron 244 processors
   6 X 1G registered ECC DDR 2100 ram
   5.1-CURRENT from Oct. 24
   Generic kernel

With all 6 sticks of memory (6G), the machine spontaneously reboots after
exactly 90 minutes of uptime.  You can set your watch by it.  There is
no panic issued, and it doesn't drop to ddb... it just reboots as if the
reset switch had been toggled.  It's only uptime that matters... I can
break to ddb, and let it sit there for hours, but when I continue, it
will reboot when the uptime hits 90 minutes.

With 4 sticks of memory (4G), it reboots after 45 minutes.  The symptoms
are the same as with 6G, except for the time.  The 4 sticks can reside in
any valid memory slot combination, it makes no difference.

With only 2 sticks of memory (2G), it is stable, and does not spontaneously
reboot.

I've currently got ACPI disabled in the bios, since I get lots of ACPI
exceptions with it enabled, but the behaviour is the same regardless of
whether or not ACPI is turned on.

We've got another similar machine running -CURRENT from Sep. 8 that does
not exhibit these problems.

There are a couple of dmesg listings available at:

   http://www.bsdwins.com/~blc/amd64/dmesg.noacpi.verbose
   http://www.bsdwins.com/~blc/amd64/dmesg.acpi

Any ideas?

Thanks,
Brad