hard-lock with CPU spinning

If I leave my laptop idle for a long period of time, it tends to lock up
with the CPU fan spinning fast (presumambly because some part of the
kernel tries to acquire a lock and spins constantly for it).

Unfortunately, this happens when X11 is running and I can't break into
DDB to snoop around.

I did notice that if I leave a terminal running:

    $ while true ; do \
          echo -n "$(date '+%Y-%m-%d %H:%M') " ; \
          tail -1 /var/log/messages ; \
          sleep 10 ; \
      done

this works as a 'workaround' for the hardlock.

The sources of /usr/src have been updated up to:

    2007.06.12.15.37.17

Any ideas how I could try to debug this?

- Giorgos