If you have problems with ACPI first check for BIOS updates.
If there's nothing available or it doesn't help, please collect
this information:

# SYS=XXXXX # replace with some descriptive name
# mkdir $SYS; cd $SYS
# acpidump -o $SYS > $SYS.aml
# dmesg > $SYS.dmesg
# cd ..
# vi $SYS.txt

Include any relevant information here, e.g. kernel output when
it crashes (see http://www.openbsd.org/faq/faq4.html#getdmesg),
a description of what happens, and your email address.

# tar czf $SYS.tgz $SYS $SYS.txt

Mail it to sthen+acpi@spacehopper.org and I'll collect them.

If you know how to build an ACPI_DEBUG kernel and capture the
output over serial console (USB serial is no good on the system
you want to capture output from, it must be a real standard
serial port; port speed higher than 9600 will save a lot of
time) then include that in a file too.