If I post to a specific list, or approve a held post for that list, there is an 80% chance that this will crash the machine (compleat lock, no interrupts, no useful log entries.

What do you mean "crash the system"? Panic and coredump? Hang? Crash mailman/python and everything else is still up?

If panic/coredump, some time spent in cursory kernel coredump analysis is warranted to point a finger at a culprit. Being a kernel developer, I'm biased, but when people say "crash the machine" that's what I think of. Is that what you mean?