Lots of messages in this forum about the spontaneous reboots of the N810 (and N900 as well) but no one has a fix for it. After three years of N810 and many firmware updates the problem still exists. Of course, the Nokia developent team als reads this forum. It seems to me that there is (for whatever reason) a random hidden crontab which reboots the device at random times.
To my experience, most reboots do not take place at heavy processor loads or hanging applications, but just in normal use.