Error: heap_mark4update: (am)invalid tid

I'm receiving an error message that I'm not sure how to resolve:
heap_mark4update: (am)invalid tid . The offending query is:

"update a set active = 'f' where a.expires < 'now' and a.active = 't' and
a.cart_id = b.cart_id"

The error is a bit inconsistent. Doing a select with the saem criteria
will work fine - but the select gives a few more rows in the result than
the update will when it works. Often (but not always), vacuuming both
tables a and b will get the update to work - but again, not always. Table
a has ~200,000 rows, and b has ~1,000 rows. Table a changes very
frequently.

The PG version is 7.3 on a dual-Athlon Linux machine. I would have
first upgraded, but I've seen references in the archives to the same (or
similar) problems happening with newer versions as well. If this is a
known issue, I'd love to hear the workaround. If not, I'll offer any
information I can to help track down the problem.

steve