Bumping this thread to post that if your N900 starts doing this, it's a very bad sign.

Mine started showing "not charging" errors when unplugged yesterday morning. In the afternoon, it started hanging for 10 seconds at a time and I knew something was wrong and tried undoing some of my OS modifications (like swap on MicroSD) to see if any were responsible, but nothing made a difference. At night it started crashing constantly. A rootfs reflash didn't fix it...soon it started boot-looping. It wouldn't accept an eMMC reflash at all and that's when I called the time of death. Luckily I have a spare that I've switched over to.