On my system, the sudden shutdowns started when I upgraded from Ubuntu
Hoary to Breezy. It looks like Breezy provides either kernel 2.6.10 or
2.6.11 (I probably had upgraded to the latter but I am not certain
now), and Breezy uses 2.6.12.
of course, I meant to write "Hoary provides either 2.6.10 or 2.6.11 and Breezy uses 2.6.12"