Postmaster Won't Start

whenever I attempt to start the postmaster, I get the
following error:

FindBackend: found "/usr/bin/postgres" using argv[0]
FATAL: StreamServerPort: bind() failed: errno=13
Is another postmaster already running on that port?
If not, remove socket node (/tmp/.s.PGSQL.<portnr>)and retry.

I'ts been working fine for several weeks until recently. There are
no files in /tmp relating to postgreSQL. The port I'm using is not in
use by another process, determined with the ps auxw|grep postma
command, and also, fuser -vun tcp 5432.

Here's the command I'm using to start postmaster:
nohup /usr/bin/postmaster -i -B 256 -p 5432 -d -D /var/lib/pgsql
>/home/postgres/pgsql.log 2>/home/postgres/pgsql.err &

I'm using postgreSQL 6.3.2 under RedHat Linux 5.1.
Any suggestions would be greatly appreciated.