I just browsed the issue where ossipena & co whose phones behave badly test with power settings. I have had mine for a week now and no reboots. I have tried chess, marbles and other things that are said to trigger this bug, but my N900 just works. It has the default settings, e.g.:
enable_off_mode 1
sleep_while_idel 1
clocks_off_while_idle 1
voltage_off_while_idle 1

I know nothing of the workings of the device, but as we all should have the same base software and some of the units reboot and some not, I would say that this is due to hardware issues. Faulty components, faulty soldering or some such. Something that happens occasionally on production line.
I would return my unit for a replacement.