invalid tid errors.

Hello,
Using Postgresql 7.3 (CVS REL7_3_STABLE today), I received the following
error:

dropsites=> delete from cart_stores;
ERROR: heap_update: (am)invalid tid

This came from a database that was dumped from 7.2.1 using 7.2.1's pg_dump
and imported into 7.3. I was able to delete the rows individually with a
where clause ( WHERE id = ... ). The problem attempting to delete all rows
persisted even when there was only 1 row left, even though this last row
deleted fine when I specified it explicitly with a where clause. I was
able to delete the contents of other tables in the db without incident. I
was also able to reproduce this on a second box, also CVS REL7_3_STABLE
from about a week ago. This problem is not exhibited under 7.2.1.

So, the question is, how do I go about tracking this down? Has anyone seen
this? It was suggested by one person that memory may be the issue, but the
fact that it is exhibited on two boxen makes that seem unlikely.

FYI:
Machine 1:
PIII 600 w/ 256MB
FreeBSD 4.6 STABLE
Machine 2:
Dual PIII 600 w/ 512MB
FreeBSD 4.7 STABLE

Thanx in advance.
-Wade Klaver