@misterc - did your problems start with the latest (21.2011.38-1Tmaemo8.3+thumb3) update? on which version you were before upgrading?

Do you overclock? Did you check filesystems for corruption(you can check fsck logs in /var/log/ )?

Sometimes reboots out of the blue happen if the battery has bad contact, you should extend the springs that hold the battery if it stands loose in the bay. Or (as pichlo said) it is just an aged battery.

Anyway, I will appreciate if you provide some more information. Could you install syslogd and sp-rich-core and provide coredump next time crash/reboot happens.

P.S. - with hundreds of cssu-thumb users, I really doubt there is some major bug passed unnoticed.