Why would a process stay in a U (uninteruptible) state?

We are running Tru64 Unix V5.1B PK5. Our own application (a relational
database) is going into a "U" state and staying there permanently. We
cannot attach to it via ladebug; cannot kill it even with a -9.

We are just looking for any ideas/suggestions as to what would cause an
application to permanently become uninterruptible. The problem has
happened 3 times in the last 5 days -- weird! The system is not paging,
not reporting hardware problems, ... it basically is behaving quite
normally except for this.

Much thanks!

Chris