How to obtain which interrupts cause system to hang?

[b. f.]

>  PID USERNAME    THR PRI NICE   SIZE    RES STATE    TIME   WCPU COMMAND
>   11 root          1 171 ki31     0K    16K RUN     24.9H 86.47% idle: cpu0
>   14 root          1 -44    -     0K    16K WAIT   689:52 10.25% swi1: net
>    2 root          1 -68    -     0K    16K sleep  207:35  4.69% ng_queue0
>   40 root          1 -68    -     0K    16K -      101:37  1.46% dummynet


It looks like there's a bit going on here: you're using dummynet and
netgraph, too?  If this setup is unstable, or otherwise problematic,
I'd suggest asking on the freebsd-net mailing list.  I don't have much
experience with some of these utilities, and I've haven't been running
FreeBSD 7-* for a long time, so I've lost track of what problems have
been fixed and back-ported.  You may also want to look at your network
and polling MIB and debugging variables, to see if they indicate any
problems.

b.