pg_clog error

Every night I pull data from a legacy system.  Last night for the first time
I got the error message:
FATAL 2:  open of /usr/local/pgsql/data/pg_clog/0081 failed: No such file or
directory
server closed the connection unexpectedly
    This probably means the server terminated abnormally
    before or while processing the request.
connection to server was lost

In the script, imports into 2 other tables, both quite large (>300k tuples)
completed successfully.  I look in the directory in question, and it is
there, but the file 0081 is not, just 0000.

This of course causes the rest of the actions in the script to fail as the
backend is restarting.  The script has never had this problem before that I
have noticed, and I confirmed that it did not happen last night.

Does anyone know what causes this?  Do I need to increase the number of file
handles somewhere?

Detailed snippet of the occurrence is below:

<snip>
psql -c 'CREATE  INDEX "customer_extra_budget_rm_idx" on
"customer_extra_budget" using btree ( "division_id" "bpchar_ops",
"elevation" "bpchar_ops", "model_id" "bpchar_ops", "option_code"
"bpchar_ops", "project_id" "bpchar_ops", "room_id" "bpchar_ops" );' -d
devtest2
CREATE
psql -c "DROP INDEX customer_extra_costs_idx; DROP INDEX
customer_extra_costs_ct_idx" -d devtest2
DROP
psql -c "delete from customer_extra_costs where division_id ='GGH';" -d
devtest2
FATAL 2:  open of /usr/local/pgsql/data/pg_clog/0081 failed: No such file or
directory
server closed the connection unexpectedly
    This probably means the server terminated abnormally
    before or while processing the request.
connection to server was lost

Thanks

Terry Fielder
Network Engineer
Great Gulf Homes / Ashton Woods Homes
terry@greatgulfhomes.com