Error during "SELECT" using psycopg 2.0.2

Hi, I'm new to this list.  I've been using psycopg for a few years
now, but have just recently begun experiencing problems with one of my
scripts that I've started using more often than I used to.

I've got a python script (python 2.4.2 on 32-bit linux) that uses
python threads and psycopg (2.0.2) to connect to postgres (8.2.3).
The threads each do a bunch of selects, inserts, updates, and deletes,
and I've started intermittently having crashes with the following
error:

Exception in thread Thread-45:
Traceback (most recent call last):
  File "/usr/lib/python2.4/threading.py", line 442, in __bootstrap
    self.run()
[snip]
OperationalError: ERROR:  SET TRANSACTION ISOLATION LEVEL must be
called before any query

The oddest thing to me is the error occurs on a SELECT statement in the form of:

SELECT col1, col2, col3 from table1 where col1='val1' and col2='val2';

...which seems like the last kind of statement that would trip an error.

Where should I look next to debug this?  As this crash happens
_sometimes_ (maybe 15% of the time), I'm having a hard time
duplicating it (which seems typical for thread-related errors).  I sit
down to debug it when I find it has happened, run the script again so
I can try to isolate the crash but the script successfully completes,
so I have to wait another day for more data to be available to
process!

~ Nathan