Hung SQL Update Linux Redhat 4U5 Postgres 8.3.1

All,

I'm hoping for some help on trying to figure out what is going on with
our postgres implementation. We have an application that uses Hibernate
to persist objects into a Postgres database. We've run into a
semi-repeatable problem (every other or every third test run) where we
issue an update SQL command to update a single row in a table and that
update just hangs (we've let it go for more than 12hrs to see if it
would eventually complete and it did not) While the update is hung the
system has a negligible CPU load (with postgres not even showing up in a
top listing)

We've dumped the locks and it shows that all locks have been granted so
it appears that it is not a lock that is standing in our way. We've
also gone in via psql while the update is hung and were able to perform
an update on the offending table without issue. Finally, we have also
enabled the statement_timeout and set it to five minutes and it does
timeout the hung update and return to normal processing by rolling back
the offending transaction but that's not a viable solution for us.

Anyone have any words of wisdom on how to track this down?
Thanks,
-Craig