Google "SmartReflex", first result:

http://focus.ti.com/general/docs/wtb...contentId=4609

It's a power-saving feature that is not stable on N900 and has been disabled by default by Nokia. They allow us to enable it if we so choose, just in case we got one of the lucky devices that doesn't suffer memory corruption and crashes due to it.

In my case, N900 reboots instantly when I enable SmartReflex.