ACPI? problem with release 8.0

[Malcolm Kay]

My machine had two SATA 300GB drives 
(WDC WD3200KS-00PFB0 21.00M21) one carrying FreeBSD RELEASE-6.3
and the other RELEASE-7.0 all of which worked OK.

Recently added SATA 1TB (WDC WD10EADS-00P8B0 01.00A01) and 
installed RELEASE 8.0 thereon. When I boot to RELEASE 8.0
I find after some time, few minutes to rather more minutes
the system just powers down without warning or any obvious cause. 
It seems to mostly happen when the system is relatively quiet.

Suspecting the ACPI I added:
 hint.acpi.0.disabled=1
to loader.conf.
I then found RELEASE 8.0 would not boot -- or at least
it was unable to mount root. I get a "mountroot>" prompt
but this seemed not to accept anything I could think of,
and "?" to list available targets yielded nothing. Rebooting and 
overriding this with option 2 (enable ACPI) in the boot menu 
took me back to a bootable but fragile system.

Changing the loader.conf entry to:
 debug.acpi.disabled=all
had the same effect as the hint.acpi.0.disabled=1.

I then thought to be somewhat selective with debug.acpi.disabled
and intended to try:
 debug.acpi.disabled=acad button cpu lid thermal timer video
only now as I write this I discover I actually entered:
 debug.acpi.disabled=acadbutton cpu lid thermal timer video

Now the RELEASE-8.0 booted but remained fragile.

I've repaired this last entry and will proceed to try it.
Meanwhile I feel I am fumbling about in the dark without
sufficient (or any real) knowledge of the range of tasks 
performed by ACPI.

Is my guess that I have an interaction problem between ACPI and
RELEASE-8.0 a reasonable one? Where can I go from here?

The system uses a Gigabyte GA-M55SLI-S4 mother board and the 
prcessor is AMD Athlon(tm) 64 X2 Dual Core Processor 5600+

Please offer suggestions or comments.

Malcolm Kay