Debugging infrequent pegged out CPU usage

I am running PostgreSQL 8.3.4 on Centos 5.2 with a single Xeon 5472,
1600 MHz, 12 MB cache, 3.0 GHz quad core, and 4 GB RAM.

My database is only about 50 MB and there are only about 20 users.

For some reason Postgres is pegging my CPU and I can barely log on to
reboot the machine. After reboot all is well for another week or so,
but this brings the system to a grinding halt.
*
What is the best way to debug this?
Can I limit Postgres to a certain number of cores or set the timeout on
the queries to a lower value?*

I would greatly appreciate any advice on debugging this problem. While
there are relatively live few users the data is extremely important and
the users will not wait for me to see what is wrong. They demand
immediate resolution and the best I can do is reboot.

--
Thank you for your time,

Jason Long
CEO and Chief Software Engineer
BS Physics, MS Chemical Engineering
http://www.octgsoftware.com
HJBug Founder and President
http://www.hjbug.com