In general random reboots can have more reasons. Apart from corrupted flash filesystem (caused by any previous random reboot in 'wrong' moment) which can be fixed by reflash there are two main issues you can't fix easily:

1. out of memory situation - enabling swap and tuning swappiness can help with this

2. wlan driver memory corruption - known for more that 1 year but only recently traced to be definitely caused by wlan driver in 770 and still unfixed - http://bugs.maemo.org/show_bug.cgi?id=2006 This one can be behind many truly random reboots as the corrupted memory page is randomly assigned to any process.