How to debug elisp memory leak

[Frode Vatvedt Fjeld]

I'm regularly getting the "lisp pointer size exceeded" warning after
about one week of running emacs. When this happens, nothing I do, such
as killing buffers, seems to help much; at most I can postpone the
inevitable crash for a few more minutes of work.

I figure there's a memory leak; somewhere there's e.g a global
variable with an ever-growing list of whatever. But I have no idea
where to start looking for what or where this might be. Can anyone
help me?

I'm using GNU Emacs 21.3.1, but this behavior has been consistent for
at least a few months and minor versions of emacs.

Thanks,
-- 
Frode Vatvedt Fjeld