Is there a specific reason you installed kernel-power? I flashed my N900 last week as well the same way you did (u-boot, bootmenu, CSSU-thumb...) with the only difference that I didn't install kernel-power.

From what I understand CSSU is providing it's own kernel that basically is like KP anyway. Yes it should work with KP but maybe in your combination/configuration there is something going wrong with it.