Cache lookup failed, and then it doesn't

Hi,

I've got a rather curious problem with my postgresql 7.3.2
installation (running on BSD on an Intel platform):

Every once in a while I receive the message 'Cache lookup failed for
relation xxxx' - even when I just try to list my tables using \d

However, when I issue the same command again - immediatly afterwards
in the same session - the query runs fine.

The entire database (30M+ records) is recreated on a daily basis
(tables truncated, vacuumed, reloaded and analyzed).

This is rather bad, as it's a production database running a telephone
directory service and sometimes gives our operators the impression
that their queries didn't find anything, and we lose a customer.

Any ideas?

Regards

Arne Wulvik