Just to make it more clear - after installing syslogd package, and running into this problem, just copy your /var/log/syslog file, upload it somewhere, and post in this thread (as link to upload, *not* whole syslog as quotation!).

As for dmesg, you don't need to install anything - you just open terminal, write dmesg, and put output here (also, important to do it quite immediately *after* you got hit by the problem).
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For the records, no such issue here,modest works fine, same for mail notifications.

/Estel

//Edit

freemangordon, how he is supposed to get dmesg output, if, when hit by the problem, he need to reboot device for getting out of it (desktop "freeze")?

I would say:
"gain ssh access from other machine before problem, and check if you can still invoke commands via ssh, when desktop is "frozen". If yes, get /var/log/syslog remotely (still via ssh, for example, copy it to microSD card inside N900), and for records, try killall hildon-desktop (to see, if desaktop comes back to life)"

Of course, as he is quite new user, it require him to install and configure OpenSSH or dropbear (which he won't regret, anyway, for other occasions).

/Estel

Now, after changing the battery, my N900 is running normal with CSSU-thumb. I think, there are stronger power-peaks under CSSU-thumb (for example while receiving Emails), as under a non-CSSU-thumb N900.

you may want to see the result of "openssl speed" for stock and -thumb device:

http://pastebin.com/Bpdt8nr7

on my phone thumb is good but sometimes some applications suddenly shuts down.i dnt hav freezing problem. the battery life is good enough now. application launch n close speed is awsome.

Highly unlikely. power regulator inside N900 works in a way, that would result in shutting down whole device (in a "bad", unmanaged way) in case of too long low voltage peak), not any software "hanging". Also, I don't think you had BME disabled - with BME working, device would shut down ("good" way, i.e. quasi-proper shutdown) mu(uuu)ch before any chance for low voltage spike, that would shut device down "bad" way.

Device "dies" of low power @ ~2850 mV. BME shut it down ~3200 mV (no hard treeshold, as it's monitoring momentary peaks, as opposed to average voltage for given second).

/Estel

One thing I have noticed is that since updating my test device, I've yet to have an app randomly close with a yellow banner. (Eg "internal error contacts application closed") I'm starting to wonder if there is a rouge library out there that Nokia made with thumb turned on that was causing random crashes before, that this errata patch now fixes.

For me the Conversations app dies randomly in that way with thumb installed. Haven't seen (or can't remember) any others, though.