panic: sleeping thread

[Paul Halliday]

I have a couple VM's that randomly halt with this error:

Sleeping thread (tid 10018, pid 1058) owns a non-sleepable lock
panic: sleeping thread
cpuid = 0
Uptime 11h14m31s
Cannot dump. Device not defined or unavailable.

FreeBSD 8.0-RELEASE FreeBSD 8.0-RELEASE #0: Sat Nov 21 15:02:08 UTC
2009     root at mason.cse.buffalo.edu:/usr/obj/usr/src/sys/GENERIC
amd64

These systems run nightly Nessus scans and these halts are very
sporadic; I can go a week w/o seeing one.

What should I do to start to troubleshoot this?

Thanks.