After reflashing both rootfs and emmc I haven't had the same instability any more for some weeks. I could shut off and turn on without the instability plaguing my device before. Everything went well since last Saturday.

At that time, I had a software crash of my n900, when the google reader app Grr with about 200 unread posts and several microb windows were opened together. The device got stuck and completely unresponsive, and I wasn't in the condition to ssh inside. My only chance was to shut it off. When on again, my optfs was clearly damaged, and the output of the autofsck started by kernel-power was lenghty and admitted, in the end, to be unable to recover the filesystem to the pristine condition.

Luckily, I was able to restore with Backupmenu, and from there on it seems stable again. Restoring the tarred image costed me 20 megabytes less in my rootfs, which is probably due to the fact that the flashed image is compressed more efficiently than the default compression of ubifs on the n900. I have 33 megabytes free, now, though and they seem to be enough.