After several tries on my N810 to fight the spontaneous reboots and tips of forum members that it is a battery problem, I finally found out this:

- The Turbo Diable kernel worsened the problem : it rebooted sometimes even at network access many times a day. So I reverted this and the reboots were less frequent.

- I tightened the battery with a piece of cardboard so that it tightly makes contact with the terminals.

- I disabled the watchdog timer with the flasher.

It still reboots 1-2 times a day even when it lies silent on the desk , no network connection or apps loaded. When it were a battery problem, a reboot could only occur at shake which may cause poor contact with the battery terminals. Even then it won't reboot but just turn off. Even a millisecond of power interruption lets it turn off. I had this issue with my Nokia 5800 phone with loose battery contacts and in all cases it turned off and did not reboot.

Does enyone have onther ideas ?

The battery-related spontaneous reboot is really a bme bug (3144/6206). An easy way to tell if you are affected by that is if your device only reboots while charging.

http://wiki.maemo.org/Troubleshooting_boot_issues has some info that could help trace the problem. Syslog (from the tools repo - remove once you're done with it as it may fill up your root fs if left unattended) might also provide some hints.

Thanks for your reply.
My N810 reboots usually when it is &not* connected to the charger. I just added some small wrapped pieces of aluminum foil in the battery contacts to improve contact, thus to minimize poor contact which may cause reboots or even turnoffs.
I did the same thing with my Nokia 5800 phone which helped against spontaneously turning off (no reboots) a year ago.

The same used to happen to me with a Mugen battery.

A new and original Nokia battery solved my problems.