Getting "cannot remove lock from HTAB" during VACUUM FULL

All,

During our weekly VACUUM FULL we get a "ERROR: cannot remove lock from HTAB"
message and then the database appears to hang completely. Other processes
can connect to the database, but all queries hang. We must stop the database
and restart it to recover. The two times this has occurred, the hang did not
happen in the same place during the VACUUM.

This behavior has only started recently; the database has been running
smoothly for over a year prior to this. We're using Postgresql 7.4.5 on an
Intel box running Red Hat AS 2.1.

Has anyone seen this before or can anyone shed some light into what might be
causing this?

Thanks,
Stuart

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