During surfing through filesysten on N800 i found an intresting (for me) folder:
/var/lib/dsme/stats
Seams like here can be found information about processes, which crash system...

In my case:
Nokia-N800-51:/var/lib/dsme/stats# cat lifeguard_resets
/usr/bin/dbus-daemon --session : 1
/usr/sbin/dsp_dld -p --disable-restart -c /lib/dsp/dsp_dld_avs.conf : 2 *

and

Nokia-N800-51:/var/lib/dsme/stats# cat lifeguard_restarts
/usr/bin/osso_hss : 7
/usr/bin/esd : 7
/usr/bin/ias : 7
/usr/bin/osso-media-server : 1
/usr/sbin/dsp_dld -p --disable-restart -c /lib/dsp/dsp_dld_avs.conf : 11
/usr/bin/matchbox-window-manager -theme default -use_titlebar yes -use_desktop_mode plain -use_lowlight no -use_cursor yes -use_dialog_mode static -use_super_modal yes : 2 *

What is dsp_dld? Is it a DSP module (driver)? What in yours files?

The asterisk is key - so in your case the window manager seemed to crash.

More information on the ReportingRebootIssues page.

Yesterday, I was having lots of problems with dsp_dld though, see #976.