Your device is trying to boot itself up after it receives some external source of power but because your device is semi-borked (you noted bootloops) this is the fundamental cause. You will need to plug your N900 into the computer and do a reflash of your device.

Each time your device reboots due to errors somewhere critical within the filesystem/kernel, your device is not getting charged enough and is rapidly draining the battery.

I would advise that the best idea is to reflash the device as soon as you can whilst your battery still holds enough charge. There would be no point in plugging N900 into a power point.