how to debug an emacs hang?

[John Owens]

I've been running the same emacs (the released version of NT emacs
21.3) for quite some time, but recently it started hanging while I'm
composing a mail message (via vm-mail), and I made it also hang when
editing a HTML file. It hangs for about a minute (to the point where
it doesn't even refresh) and eventually all my text suddenly appears
and it's all back to normal. Hitting C-g does not seem to do anything
while it is hanging.

I'd like to figure out how/where it's hanging, but I don't know how I
might do so. Suggestions?

JDO