HP DL380 hangs on reboot

I have an HP DL380 running 5.3 and it will not reboot from multi-user
mode - it hangs after printing "Rebooting..." and needs to be power-
cycled (since there's no reset button).

This doesn't happen with 4.10 or in single-user mode.  ACPI and
BROKEN_KEYBOARD_RESET make no difference.  Adding some printf's in the
shutdown sequence shows that the code is getting to the keyboard reset
or invltlb() correctly.

The big difference between 4.10 and 5.3 is ACPI - but disabling it
doesn't have any effect.  Does anyone have any ideas for where to
search next?  Working through /etc/rc.d would be a long and painful
process since the POST takes a couple of minutes.

BTW, would someone like to either explain or provide a pointer to how
zeroing the page table and wiping the TLB resets a PC.  I would have
expected the CPU to wedge when it can't access memory (it can't take
an exception because it can't save the state or fetch the exception
descriptor).

-- 
Peter Jeremy