NMI on shutdown

Hi,

 The following log messages are displayed on a box where I am testing
 stable/9.  It occurs only when trying to shutdown the box:

 | Waiting (max 60 seconds) for system process `vnlru' to stop...done
 | Waiting (max 60 seconds) for system process `bufdaemon' to stop...done
 | Waiting (max 60 seconds) for system process `syncer' to stop...
 | Syncing disks, vnodes remaining...2 2 0 0 done
 | All buffers synced.
 | Uptime: 8d2h4m58s
 | NMI ISA 30, EISA 0
 | NMI ISA 20, EISA 0
 | NMI ... going to debugger
 | NMI ... going to debugger
 | NMI ISA 20, EISA 0
 | NMI ISA 30, EISA 0
 | NMI ... going to debugger
 | NMI ... going to debugger
 | NMI ISA 20, EISA 0
 ...
 | timeout stopping cpus
 | [ thread pid 11 tid 100016 ]
 | Stopped at      acpi_cpu_c1+0x6:        leave
 | db> timeout stopping cpus
 | timeout stopping cpus
 | [ thread pid 11 tid 100015 ]
 | Stopped at      acpi_cpu_c1+0x6:        leave
 | db> timeout stopping cpus
 | timeout stopping cpus

 Once these are displayed, a power cycle is required to recover.
 Shutdown sometimes works, and sometimes not.  DDB prompt does not
 work because it does not accept entered characters properly.  And,
 this symptom seems not specific to stable/9.

 This may be a hardware specific issue, but where should I start to
 debug this from?

-- Hiroki