Is your N900 powering off unexpectedly ? when you:
* run with your device in your pocket
* close the keyboard
* shake your device (it's not a feature)

well, don't be like me, i opened the device and lost one screw, checked the ribbon between the screen and the motherboard... etc

but the answer was easier:

just check that your battery pins are narrow enough to hold tight the N900 connectors to the battery , if not you can buy a new battery or use the tiny screwdriver and try to narrow the space between the pins.

worked great for me

Haha, it's quite old thing, and I'm laughing, cause I remember when I was scratching my head about "why my device power down on keyboard open/close".

I was almost do desperate to make a reflash - fortunately, somehow I got an idea of checking battery pins. One of the "oh, sh|t!" moments.

Anyway, it was mentioned many times before, and I'm not sure if it require separate thread. dr_frost_dk's battery related " battery tests and3000 mAh battery mod" (see my signature for link) seems to be perfect place to put such advice.