Frequent pg_clog on system

Hi all,

I'm using postgresql 8.1.8 on Ubuntu 6.10, it's mainly a
sandbox/playground machine to crunch data and do some stats analysis.
System info:
2.6.17-11-server (SMP)
Intel(R) Pentium(R) 4 CPU 3.00GHz
4GB RAM
500GB disk

Size of the db is ~7GB with 16 tables, 2 tables A and B holding
respectively 4GB (6M rows) and 2GB(3M rows) of data
I do at this time about 600K insert / day in one go.
In some occasion there is a lot of duplicate inserts (the data I get, is
sometimes duplicated and I rely on unique constraint)

Since I started to play with it, I got too many times the pg_clog error
such as:
ERROR: could not access status of transaction 778793573
DETAIL: could not open file "pg_clog/02E6": No such file or directory

Each time the pg_clog file is way beyond the existing files I have on my
system (in this case, the 'latest file' is 0023)

It seems to happen after a sql query performs a long scan of table A or B.
That's very weird since each time it seems to happen after performing
more or less the same query several times (trial and error narrowing
them data) and suddenly I have this error happening and I'm cooked
(fwiw, there is no insert/update happening at all during this period,
only select and a single user)

Anyone has any idea what could be the cause of this ? It's a bit too
recurrent to me and it's a definite annoyance.

Cheers,

-- stephane