Idle emacs taking 100% cpu - how to track it?

[Marcin Kasperski]

For some time I observe some irritating problem: when I leave some
emacs running (say, on another virtual desktop) and unused, after some
time it starts taking 100% CPU (on the fast modern machine). Sometimes
it is still responsive and I can start using it again (while it
handles keystrokes it usually stop eating CPU but when I leave it it
starts hogging again), sometimes it seems frozen (does not repaint
buffer windows).

I use debian's snapshot version (at the moment GNU Emacs 22.0.95.1
(i486-pc-linux-gnu, GTK+ Version 2.8.20))

First I suspected server mode, but replacing it with gnuserv did not
change anything.

Now I suspect 'something' updating semantic.cache (it happened to me
that when I had returned to hogging emacs, found it frozen and
pressed Ctrl-G, after some time semantic.cache has been displayed).
But I am not sure.

Has anybody faced similar behaviour?

What can I do to track the problem (I of course suspect some elisp
code going wild, but how to check what exactly is wrong...)?