Hi I had the same problem before some weeks,so what i did:
sudo gainroot
dpkg -l | grep ^rc
dpkg -l | grep ^rc | cut -d " " -f 3 | xargs dpkg --purge
removing kp52:
apt-get remove -y kernel-power kernel-power-modules
dpkg -l | grep ^rc
dpkg -l | grep ^rc | cut -d " " -f 3 | xargs dpkg --purge
apt-get clean
apt-get autoclean
apt-get autoremove
don't touch the icon from menu
restart n900,if you don't boot the system flash only kernel:
flasher-3.5 -F RX-51_2009SE_20.2010.36-2_PR_COMBINED_MR0_ARM.bin --flash-only=kernel -f -R
Add the sdk repositores of fremantle in /etc/apt/sources.list with opensshserver:
Install u-boot from pali;
apt-get install -y u-boot-tools u-boot-flasher
restart n900
apt-get install kernel-power-settings
restart n900
sudo gainroot
u-boot-update-bootmenu
restart with open ukeyboard
choose kp52
If you want to installing backupmenu:
apt-get install backupmenu
restart n900,with close ukeyboard,when shows the titles of u-boot,open ukeyboard and you see how loading bootmenu,choose maemo with..
then advice from Pali:
root
ln -s /etc/bootmenu.d/20-Maemo5-kernel-power-2.6.28.10-power52.item /etc/default/bootmenu.item
after that the backmenu shoud properly load kp52
I think this is all,if somebody have other advice please share it
edit:Opps sorry man,that way is not for you because you have cssu-thumb and if you remove kp52 then you have to complete flashing your n900,again sorry man i did't notice than you are with cssu-thumb