corrupted item pointer:???

hi.
basic information:
machine: desktop computer, with sata hard drive, no bad blocks. 2g
ram.AMDSempron(tm) Processor 2600+
system: linux debian testing, using 2.6.11 kernel.
postgresql: 8.1.3 compiled by hand using:
./configure \
--prefix=/home/pgdba/work \
--without-debug \
--disable-debug \
--with-pgport=5810 \
--with-tcl \
--with-perl \
--with-python \
--without-krb4 \
--without-krb5 \
--without-pam \
--without-rendezvous \
--with-openssl \
--with-readline \
--with-zlib \
--with-gnu-ld

version() -> PostgreSQL 8.1.3 on i686-pc-linux-gnu, compiled by GCC gcc
(GCC) 3.3.5 (Debian 1:3.3.5-13)

on this machine i have copy of 40G database from production servers.
i was testing migration to hstore (
http://www.sai.msu.su/~megera/postgres/gist/hstore/README.hstore)

at some point ot tests it failed saying corrupted item pointer.

migration was done using:
LOOP:
select data from test_table where primary_key = constant;
select data from secondary_table where unique_key = constant;
update test_table set hstore_field = some_value where primary_key =
constant
REPEAT FOR ALL items;
this loop was made in external code (perl, using dbi + dbd::pg).
we did about 37 such loops every second.
everything went fine.
then - every 30 minutes) i ran vacuum to reclaim space from test_table.
at one of such vacuums it paniced showing me forementioned error and killing
all connections.

is there any way i can check what went wrong?
i dont need to recover the data.
i just need to know wherher the problem is hstore-related, hardware-related
or just random thing happening because of nothing.

i tried:
hand-vacuum
vacuum analyze
vacuum full analyze
reindex the table
another vacuum
none of this worked.
i still get the corrupted item pointer message.

any clues on how can i check what went wrong?

depesz