It might be easier in order to troubleshoot the issue using ssh sessions.

I would advise to open one for each of the following commands to start with:

vmstat 1
top -b
tail -f /var/log/messages
tail -f /var/log/kernel
(not sure about the exact localisation of these two last log files as I don't own yet a n900... yet... but if you provide a 'find /var/log' we should be able to figure that out)

Another alternative to SSH will be to use the usb interface as tty interface but I don't know if this is supported out of the box.
The main reason for the ssh/serial interface is that the reboot may not sync the files to the disk and therefore never troubleshoot the root of the issue but will be visible on the session opened.