HELP: pg_clog file not found error

When my server is under heavy load I sometimes get this error (see below),
and the backend restarts.

Does anyone know what causes this? Could it be a max number of filehandles
reached? If so how do I increase it? (I know I have lots of disk space on
that partition)

I am running postgres: postmaster (PostgreSQL) 7.2.1
on Redhat 6.2 with the original (inital) kernel.

Any ideas greatly appreciated.

Thanks

...
DEBUG: query: select btrim($1, ' ')
DEBUG: query: select btrim($1, ' ')
FATAL 2: open of /usr/local/pgsql/data/pg_clog/0522 failed: No such file or
dir
ectory
DEBUG: proc_exit(2)
DEBUG: shmem_exit(2)
DEBUG: exit(2)
DEBUG: reaping dead processes
DEBUG: child process (pid 2432) exited with exit code 2
DEBUG: server process (pid 2432) exited with exit code 2
DEBUG: terminating any other active server processes
DEBUG: CleanupProc: sending SIGQUIT to process 2520
DEBUG: CleanupProc: sending SIGQUIT to process 2395
NOTICE: Message from PostgreSQL backend:
The Postmaster has informed me that some other backend
died abnormally and possibly corrupted shared memory.
I have rolled back the current transaction and am
going to terminate your database system connection and exit.
Please reconnect to the database system and repeat your query.
DEBUG: reaping dead processes

Terry Fielder
Manager Software Development and Deployment
Great Gulf Homes / Ashton Woods Homes
terry(at)greatgulfhomes(dot)com